Главная Обзор Помощь
Регистрация Вход
root
/
blog_web
1
0
Ответвить 0
Файлы Задачи 0 Запросы на слияние 0 Вики
Дерево: 4bd31e0f84
Ветки Метки
blog_web
/
pages
/
efe2fb04eb8ac5fb
/
index.html

index.html 136 KB
История Исходник

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383 
  1. <!DOCTYPE html>
    2. 

  2. <html lang="zh-CN">
    3. 

  3.   <head>
    4. 

  4.     <meta charset="utf-8">
    5. 

  5.     <meta name="viewport" content="width=device-width,initial-scale=1">
    6. 

  6.     <title>Module 的语法 | 彪哥博客</title>
    7. 

  7.     <meta name="generator" content="VuePress 1.9.5">
    8. 

  8.     <link rel="icon" href="/blog/img/favicon.ico">
    9. 

  9.     <meta name="description" content="web前端技术博客,专注web前端学习与总结。JavaScript,js,ES6,TypeScript,vue,React,python,css3,html5,Node,git,github等技术文章。">
    10. 

  10.     <meta name="keywords" content="前端博客,个人技术博客,前端,前端开发,前端框架,web前端,前端面试题,技术文档,学习,面试,JavaScript,js,ES6,TypeScript,vue,python,css3,html5,Node,git,github,markdown">
    11. 

  11.     <meta name="theme-color" content="#11a8cd">
    12. 

  12.     
    13. 

  13.     <link rel="preload" href="/blog/assets/css/0.styles.cf8cd190.css" as="style"><link rel="preload" href="/blog/assets/js/app.68c9df80.js" as="script"><link rel="preload" href="/blog/assets/js/2.395c0d18.js" as="script"><link rel="preload" href="/blog/assets/js/3.6748bd5c.js" as="script"><link rel="preload" href="/blog/assets/js/110.f41596b0.js" as="script"><link rel="prefetch" href="/blog/assets/js/10.cad3aa70.js"><link rel="prefetch" href="/blog/assets/js/100.08a8b2d8.js"><link rel="prefetch" href="/blog/assets/js/101.2aabb12c.js"><link rel="prefetch" href="/blog/assets/js/102.13f3cc4d.js"><link rel="prefetch" href="/blog/assets/js/103.c19aee03.js"><link rel="prefetch" href="/blog/assets/js/104.91a6aec1.js"><link rel="prefetch" href="/blog/assets/js/105.c39c4cd9.js"><link rel="prefetch" href="/blog/assets/js/106.c34bb1d8.js"><link rel="prefetch" href="/blog/assets/js/107.4a53669b.js"><link rel="prefetch" href="/blog/assets/js/108.a43622c2.js"><link rel="prefetch" href="/blog/assets/js/109.3219d87b.js"><link rel="prefetch" href="/blog/assets/js/11.f2e9eca8.js"><link rel="prefetch" href="/blog/assets/js/111.da99a105.js"><link rel="prefetch" href="/blog/assets/js/112.9dd75c6f.js"><link rel="prefetch" href="/blog/assets/js/113.9322f157.js"><link rel="prefetch" href="/blog/assets/js/114.c5150cc6.js"><link rel="prefetch" href="/blog/assets/js/115.f58b49f8.js"><link rel="prefetch" href="/blog/assets/js/116.8f7d9c6f.js"><link rel="prefetch" href="/blog/assets/js/117.3c970f48.js"><link rel="prefetch" href="/blog/assets/js/118.921e1d54.js"><link rel="prefetch" href="/blog/assets/js/119.0141defb.js"><link rel="prefetch" href="/blog/assets/js/12.98512c60.js"><link rel="prefetch" href="/blog/assets/js/120.de47a761.js"><link rel="prefetch" href="/blog/assets/js/121.a0b3693a.js"><link rel="prefetch" href="/blog/assets/js/122.6c7dd225.js"><link rel="prefetch" href="/blog/assets/js/123.dbff103c.js"><link rel="prefetch" href="/blog/assets/js/124.493776ef.js"><link rel="prefetch" href="/blog/assets/js/125.f09bc798.js"><link rel="prefetch" href="/blog/assets/js/126.3e28e8a2.js"><link rel="prefetch" href="/blog/assets/js/127.aad20a7e.js"><link rel="prefetch" href="/blog/assets/js/128.6543adba.js"><link rel="prefetch" href="/blog/assets/js/129.d7c56b92.js"><link rel="prefetch" href="/blog/assets/js/13.a79fa0c7.js"><link rel="prefetch" href="/blog/assets/js/130.593d21f0.js"><link rel="prefetch" href="/blog/assets/js/131.4c90d8b8.js"><link rel="prefetch" href="/blog/assets/js/132.4ad12bdc.js"><link rel="prefetch" href="/blog/assets/js/133.485de1b9.js"><link rel="prefetch" href="/blog/assets/js/134.78bc8f57.js"><link rel="prefetch" href="/blog/assets/js/135.47498729.js"><link rel="prefetch" href="/blog/assets/js/136.d99350df.js"><link rel="prefetch" href="/blog/assets/js/137.35d58a22.js"><link rel="prefetch" href="/blog/assets/js/138.75e29c6f.js"><link rel="prefetch" href="/blog/assets/js/139.5af059c1.js"><link rel="prefetch" href="/blog/assets/js/14.0fdf0c78.js"><link rel="prefetch" href="/blog/assets/js/140.0d9b8fbc.js"><link rel="prefetch" href="/blog/assets/js/141.128b6e26.js"><link rel="prefetch" href="/blog/assets/js/142.d7c4dea7.js"><link rel="prefetch" href="/blog/assets/js/143.9aa933f3.js"><link rel="prefetch" href="/blog/assets/js/144.277e558e.js"><link rel="prefetch" href="/blog/assets/js/145.3b994de3.js"><link rel="prefetch" href="/blog/assets/js/146.24441c89.js"><link rel="prefetch" href="/blog/assets/js/147.f7f1952e.js"><link rel="prefetch" href="/blog/assets/js/148.25581f95.js"><link rel="prefetch" href="/blog/assets/js/149.882bdb5a.js"><link rel="prefetch" href="/blog/assets/js/15.fce722b2.js"><link rel="prefetch" href="/blog/assets/js/150.dc1a32f0.js"><link rel="prefetch" href="/blog/assets/js/151.250a4894.js"><link rel="prefetch" href="/blog/assets/js/152.fa700729.js"><link rel="prefetch" href="/blog/assets/js/153.b007c8a2.js"><link rel="prefetch" href="/blog/assets/js/154.8ab912b6.js"><link rel="prefetch" href="/blog/assets/js/155.5ee5b308.js"><link rel="prefetch" href="/blog/assets/js/156.906dc55e.js"><link rel="prefetch" href="/blog/assets/js/157.08c6766f.js"><link rel="prefetch" href="/blog/assets/js/158.53ad5a6c.js"><link rel="prefetch" href="/blog/assets/js/159.a83db4c9.js"><link rel="prefetch" href="/blog/assets/js/16.7cf1b239.js"><link rel="prefetch" href="/blog/assets/js/160.4cded216.js"><link rel="prefetch" href="/blog/assets/js/161.9471d56d.js"><link rel="prefetch" href="/blog/assets/js/162.0c32d380.js"><link rel="prefetch" href="/blog/assets/js/163.41d60a5c.js"><link rel="prefetch" href="/blog/assets/js/164.6dbc8ae8.js"><link rel="prefetch" href="/blog/assets/js/165.7e1b67fe.js"><link rel="prefetch" href="/blog/assets/js/166.ca626fb4.js"><link rel="prefetch" href="/blog/assets/js/167.2f20b6fa.js"><link rel="prefetch" href="/blog/assets/js/168.2e605db8.js"><link rel="prefetch" href="/blog/assets/js/169.401b96d4.js"><link rel="prefetch" href="/blog/assets/js/17.cea117e8.js"><link rel="prefetch" href="/blog/assets/js/170.7c8b0366.js"><link rel="prefetch" href="/blog/assets/js/171.c3155533.js"><link rel="prefetch" href="/blog/assets/js/172.b659d767.js"><link rel="prefetch" href="/blog/assets/js/173.62c681db.js"><link rel="prefetch" href="/blog/assets/js/174.5c66f092.js"><link rel="prefetch" href="/blog/assets/js/175.bbbdffbe.js"><link rel="prefetch" href="/blog/assets/js/176.262d26df.js"><link rel="prefetch" href="/blog/assets/js/177.e006c7d1.js"><link rel="prefetch" href="/blog/assets/js/178.8b7976a5.js"><link rel="prefetch" href="/blog/assets/js/179.cf34a2b2.js"><link rel="prefetch" href="/blog/assets/js/18.c338fe95.js"><link rel="prefetch" href="/blog/assets/js/180.d2a4e612.js"><link rel="prefetch" href="/blog/assets/js/181.9338fd07.js"><link rel="prefetch" href="/blog/assets/js/182.e83e4a62.js"><link rel="prefetch" href="/blog/assets/js/183.472c384f.js"><link rel="prefetch" href="/blog/assets/js/184.711e54a6.js"><link rel="prefetch" href="/blog/assets/js/185.20075148.js"><link rel="prefetch" href="/blog/assets/js/186.08c67f20.js"><link rel="prefetch" href="/blog/assets/js/187.0aa1cef8.js"><link rel="prefetch" href="/blog/assets/js/188.5d767d7a.js"><link rel="prefetch" href="/blog/assets/js/189.e8e2eb21.js"><link rel="prefetch" href="/blog/assets/js/19.6b963460.js"><link rel="prefetch" href="/blog/assets/js/190.8b557318.js"><link rel="prefetch" href="/blog/assets/js/191.3e6fdc4f.js"><link rel="prefetch" href="/blog/assets/js/192.99d6e1d8.js"><link rel="prefetch" href="/blog/assets/js/193.4ced091e.js"><link rel="prefetch" href="/blog/assets/js/194.4b375e2e.js"><link rel="prefetch" href="/blog/assets/js/195.003e3d67.js"><link rel="prefetch" href="/blog/assets/js/196.fe952fa6.js"><link rel="prefetch" href="/blog/assets/js/197.ff421a66.js"><link rel="prefetch" href="/blog/assets/js/198.628c2c1a.js"><link rel="prefetch" href="/blog/assets/js/199.89c4e586.js"><link rel="prefetch" href="/blog/assets/js/20.839dae41.js"><link rel="prefetch" href="/blog/assets/js/200.1d9f2ef9.js"><link rel="prefetch" href="/blog/assets/js/201.5ba078d9.js"><link rel="prefetch" href="/blog/assets/js/202.b49b23f4.js"><link rel="prefetch" href="/blog/assets/js/203.d88a03d9.js"><link rel="prefetch" href="/blog/assets/js/204.145e5762.js"><link rel="prefetch" href="/blog/assets/js/205.5a01e256.js"><link rel="prefetch" href="/blog/assets/js/206.5da6ab5c.js"><link rel="prefetch" href="/blog/assets/js/207.02a926dc.js"><link rel="prefetch" href="/blog/assets/js/208.40276146.js"><link rel="prefetch" href="/blog/assets/js/209.edc34975.js"><link rel="prefetch" href="/blog/assets/js/21.bd21bd29.js"><link rel="prefetch" href="/blog/assets/js/210.02e4ee2f.js"><link rel="prefetch" href="/blog/assets/js/211.0f8a9304.js"><link rel="prefetch" href="/blog/assets/js/212.9220dd3d.js"><link rel="prefetch" href="/blog/assets/js/213.89521ebd.js"><link rel="prefetch" href="/blog/assets/js/214.cd5ec468.js"><link rel="prefetch" href="/blog/assets/js/215.0084d772.js"><link rel="prefetch" href="/blog/assets/js/216.ebd468b9.js"><link rel="prefetch" href="/blog/assets/js/217.fac21407.js"><link rel="prefetch" href="/blog/assets/js/218.9793e19a.js"><link rel="prefetch" href="/blog/assets/js/219.c89175e9.js"><link rel="prefetch" href="/blog/assets/js/22.e6bcf65d.js"><link rel="prefetch" href="/blog/assets/js/220.d496e411.js"><link rel="prefetch" href="/blog/assets/js/221.07a9338a.js"><link rel="prefetch" href="/blog/assets/js/222.22bf261f.js"><link rel="prefetch" href="/blog/assets/js/223.b9c6a055.js"><link rel="prefetch" href="/blog/assets/js/224.e29550d2.js"><link rel="prefetch" href="/blog/assets/js/225.a406a2ff.js"><link rel="prefetch" href="/blog/assets/js/226.78192713.js"><link rel="prefetch" href="/blog/assets/js/227.900f2296.js"><link rel="prefetch" href="/blog/assets/js/228.7243da66.js"><link rel="prefetch" href="/blog/assets/js/229.009debcf.js"><link rel="prefetch" href="/blog/assets/js/23.ebbc3fd3.js"><link rel="prefetch" href="/blog/assets/js/230.109a1752.js"><link rel="prefetch" href="/blog/assets/js/231.55d62061.js"><link rel="prefetch" href="/blog/assets/js/232.049029b8.js"><link rel="prefetch" href="/blog/assets/js/233.42dfc185.js"><link rel="prefetch" href="/blog/assets/js/234.4dbf5d59.js"><link rel="prefetch" href="/blog/assets/js/235.f78274d9.js"><link rel="prefetch" href="/blog/assets/js/236.a2b61bec.js"><link rel="prefetch" href="/blog/assets/js/237.6ae31c88.js"><link rel="prefetch" href="/blog/assets/js/238.2f5c56ae.js"><link rel="prefetch" href="/blog/assets/js/239.506a4e9f.js"><link rel="prefetch" href="/blog/assets/js/24.19bd04ec.js"><link rel="prefetch" href="/blog/assets/js/25.b4de33d1.js"><link rel="prefetch" href="/blog/assets/js/26.d9940ceb.js"><link rel="prefetch" href="/blog/assets/js/27.83d848ee.js"><link rel="prefetch" href="/blog/assets/js/28.31289bac.js"><link rel="prefetch" href="/blog/assets/js/29.45af5621.js"><link rel="prefetch" href="/blog/assets/js/30.d5c08e66.js"><link rel="prefetch" href="/blog/assets/js/31.78e43a68.js"><link rel="prefetch" href="/blog/assets/js/32.53ca76ee.js"><link rel="prefetch" href="/blog/assets/js/33.081e8ef6.js"><link rel="prefetch" href="/blog/assets/js/34.cb1866c1.js"><link rel="prefetch" href="/blog/assets/js/35.dca9b927.js"><link rel="prefetch" href="/blog/assets/js/36.0cee37e8.js"><link rel="prefetch" href="/blog/assets/js/37.637acdf4.js"><link rel="prefetch" href="/blog/assets/js/38.6624bf02.js"><link rel="prefetch" href="/blog/assets/js/39.58eab309.js"><link rel="prefetch" href="/blog/assets/js/4.44654b1a.js"><link rel="prefetch" href="/blog/assets/js/40.00ca637c.js"><link rel="prefetch" href="/blog/assets/js/41.2b5e8c27.js"><link rel="prefetch" href="/blog/assets/js/42.350d128c.js"><link rel="prefetch" href="/blog/assets/js/43.871dce82.js"><link rel="prefetch" href="/blog/assets/js/44.6d89c3e1.js"><link rel="prefetch" href="/blog/assets/js/45.6e554c79.js"><link rel="prefetch" href="/blog/assets/js/46.d15a7dc0.js"><link rel="prefetch" href="/blog/assets/js/47.8d66ca97.js"><link rel="prefetch" href="/blog/assets/js/48.3c1102e1.js"><link rel="prefetch" href="/blog/assets/js/49.e17a3436.js"><link rel="prefetch" href="/blog/assets/js/5.88de390f.js"><link rel="prefetch" href="/blog/assets/js/50.451b1c8b.js"><link rel="prefetch" href="/blog/assets/js/51.3799fbac.js"><link rel="prefetch" href="/blog/assets/js/52.f3ef3b5e.js"><link rel="prefetch" href="/blog/assets/js/53.a6bacd25.js"><link rel="prefetch" href="/blog/assets/js/54.dbb7c9ab.js"><link rel="prefetch" href="/blog/assets/js/55.2562d0c8.js"><link rel="prefetch" href="/blog/assets/js/56.14ea4931.js"><link rel="prefetch" href="/blog/assets/js/57.268c586a.js"><link rel="prefetch" href="/blog/assets/js/58.88fed51b.js"><link rel="prefetch" href="/blog/assets/js/59.02b32879.js"><link rel="prefetch" href="/blog/assets/js/6.277038ca.js"><link rel="prefetch" href="/blog/assets/js/60.a27059ce.js"><link rel="prefetch" href="/blog/assets/js/61.39fb215e.js"><link rel="prefetch" href="/blog/assets/js/62.0210588e.js"><link rel="prefetch" href="/blog/assets/js/63.05521a18.js"><link rel="prefetch" href="/blog/assets/js/64.8aa613a8.js"><link rel="prefetch" href="/blog/assets/js/65.acfdd371.js"><link rel="prefetch" href="/blog/assets/js/66.cb805d9b.js"><link rel="prefetch" href="/blog/assets/js/67.39f85baa.js"><link rel="prefetch" href="/blog/assets/js/68.7f79766a.js"><link rel="prefetch" href="/blog/assets/js/69.fa8624bd.js"><link rel="prefetch" href="/blog/assets/js/7.474f8f8d.js"><link rel="prefetch" href="/blog/assets/js/70.1f3e978d.js"><link rel="prefetch" href="/blog/assets/js/71.13cd9358.js"><link rel="prefetch" href="/blog/assets/js/72.739b22a8.js"><link rel="prefetch" href="/blog/assets/js/73.95f69ae2.js"><link rel="prefetch" href="/blog/assets/js/74.b6624f6a.js"><link rel="prefetch" href="/blog/assets/js/75.b0d9aa06.js"><link rel="prefetch" href="/blog/assets/js/76.18ef5b96.js"><link rel="prefetch" href="/blog/assets/js/77.46f6e413.js"><link rel="prefetch" href="/blog/assets/js/78.aebd00ee.js"><link rel="prefetch" href="/blog/assets/js/79.1b784d15.js"><link rel="prefetch" href="/blog/assets/js/8.9428e7ee.js"><link rel="prefetch" href="/blog/assets/js/80.1f550d53.js"><link rel="prefetch" href="/blog/assets/js/81.101cc131.js"><link rel="prefetch" href="/blog/assets/js/82.077c8298.js"><link rel="prefetch" href="/blog/assets/js/83.2e375d11.js"><link rel="prefetch" href="/blog/assets/js/84.38102a34.js"><link rel="prefetch" href="/blog/assets/js/85.24532d6a.js"><link rel="prefetch" href="/blog/assets/js/86.1dabbf00.js"><link rel="prefetch" href="/blog/assets/js/87.9ae389e9.js"><link rel="prefetch" href="/blog/assets/js/88.10e8abd6.js"><link rel="prefetch" href="/blog/assets/js/89.187e5e16.js"><link rel="prefetch" href="/blog/assets/js/9.da143545.js"><link rel="prefetch" href="/blog/assets/js/90.96496e91.js"><link rel="prefetch" href="/blog/assets/js/91.e7933218.js"><link rel="prefetch" href="/blog/assets/js/92.5484868f.js"><link rel="prefetch" href="/blog/assets/js/93.c8ee75e3.js"><link rel="prefetch" href="/blog/assets/js/94.f70985e2.js"><link rel="prefetch" href="/blog/assets/js/95.3abe7fbe.js"><link rel="prefetch" href="/blog/assets/js/96.80e5a938.js"><link rel="prefetch" href="/blog/assets/js/97.1f5e5197.js"><link rel="prefetch" href="/blog/assets/js/98.e3a275c8.js"><link rel="prefetch" href="/blog/assets/js/99.d33bf89e.js">
    14. 

  14.     <link rel="stylesheet" href="/blog/assets/css/0.styles.cf8cd190.css">
    15. 

  15.   </head>
    16. 

  16.   <body class="theme-mode-light">
    17. 

  17.     <div id="app" data-server-rendered="true"><div class="theme-container sidebar-open have-rightmenu"><header class="navbar blur"><div title="目录" class="sidebar-button"><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" role="img" viewBox="0 0 448 512" class="icon"><path fill="currentColor" d="M436 124H12c-6.627 0-12-5.373-12-12V80c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12z"></path></svg></div> <a href="/blog/" class="home-link router-link-active"><img src="/blog/img/logo.png" alt="彪哥博客" class="logo"> <span class="site-name can-hide">彪哥博客</span></a> <div class="links"><div class="search-box"><input aria-label="Search" autocomplete="off" spellcheck="false" value=""> <!----></div> <nav class="nav-links can-hide"><div class="nav-item"><a href="/blog/" class="nav-link">首页</a></div><div class="nav-item"><a href="http://fseller.com" target="_blank" rel="noopener noreferrer" class="nav-link external">
    18. 

  18.   个人游戏网站
    19. 

  19.   <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/blog/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/note/javascript/" class="nav-link">《JavaScript教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/js/" class="nav-link">《JavaScript高级程序设计》</a></li><li class="dropdown-subitem"><a href="/blog/note/es6/" class="nav-link">《ES6 教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/vue/" class="nav-link">《Vue》</a></li><li class="dropdown-subitem"><a href="/blog/note/react/" class="nav-link">《React》</a></li><li class="dropdown-subitem"><a href="/blog/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/blog/note/git/" class="nav-link">《Git》</a></li><li class="dropdown-subitem"><a href="/blog/pages/51afd6/" class="nav-link">TypeScript</a></li><li class="dropdown-subitem"><a href="/blog/pages/4643cd/" class="nav-link">JS设计模式总结</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/blog/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/blog/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="更多" class="dropdown-title"><a href="/blog/more/" class="link-title">更多</a> <span class="title" style="display:none;">更多</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/f2a556/" class="nav-link">学习</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/aea6571b7a8bae86/" class="nav-link">面试</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/2d615df9a36a98ed/" class="nav-link">心情杂货</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/baaa02/" class="nav-link">实用技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/friends/" class="nav-link">友情链接</a></li></ul></div></div><div class="nav-item"><a href="/blog/about/" class="nav-link">关于</a></div><div class="nav-item"><a href="/blog/pages/beb6c0bd8a66cea6/" class="nav-link">收藏</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="索引" class="dropdown-title"><a href="/blog/archives/" class="link-title">索引</a> <span class="title" style="display:none;">索引</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/categories/" class="nav-link">分类</a></li><li class="dropdown-item"><!----> <a href="/blog/tags/" class="nav-link">标签</a></li><li class="dropdown-item"><!----> <a href="/blog/archives/" class="nav-link">归档</a></li></ul></div></div> <a href="https://github.com/heBody/blog" target="_blank" rel="noopener noreferrer" class="repo-link">
    20. 

  20.     GitHub
    21. 

  21.     <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav></div></header> <div class="sidebar-mask"></div> <div class="sidebar-hover-trigger"></div> <aside class="sidebar" style="display:none;"><div class="blogger"><img src="https://fastly.jsdelivr.net/gh/xugaoyi/image_store/blog/20200103123203.jpg"> <div class="blogger-info"><h3>彪哥</h3> <span>爱好前端</span></div></div> <nav class="nav-links"><div class="nav-item"><a href="/blog/" class="nav-link">首页</a></div><div class="nav-item"><a href="http://fseller.com" target="_blank" rel="noopener noreferrer" class="nav-link external">
    22. 

  22.   个人游戏网站
    23. 

  23.   <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="前端" class="dropdown-title"><a href="/blog/web/" class="link-title">前端</a> <span class="title" style="display:none;">前端</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>前端文章</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/pages/8143cc480faf9a11/" class="nav-link">JavaScript</a></li></ul></li><li class="dropdown-item"><h4>学习笔记</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/blog/note/javascript/" class="nav-link">《JavaScript教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/js/" class="nav-link">《JavaScript高级程序设计》</a></li><li class="dropdown-subitem"><a href="/blog/note/es6/" class="nav-link">《ES6 教程》</a></li><li class="dropdown-subitem"><a href="/blog/note/vue/" class="nav-link">《Vue》</a></li><li class="dropdown-subitem"><a href="/blog/note/react/" class="nav-link">《React》</a></li><li class="dropdown-subitem"><a href="/blog/note/typescript-axios/" class="nav-link">《TypeScript 从零实现 axios》</a></li><li class="dropdown-subitem"><a href="/blog/note/git/" class="nav-link">《Git》</a></li><li class="dropdown-subitem"><a href="/blog/pages/51afd6/" class="nav-link">TypeScript</a></li><li class="dropdown-subitem"><a href="/blog/pages/4643cd/" class="nav-link">JS设计模式总结</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="页面" class="dropdown-title"><a href="/blog/ui/" class="link-title">页面</a> <span class="title" style="display:none;">页面</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/8309a5b876fc95e3/" class="nav-link">HTML</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/0a83b083bdf257cb/" class="nav-link">CSS</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="技术" class="dropdown-title"><a href="/blog/technology/" class="link-title">技术</a> <span class="title" style="display:none;">技术</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/9a7ee40fc232253e/" class="nav-link">技术文档</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/4c778760be26d8b3/" class="nav-link">GitHub技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/117708e0af7f0bd9/" class="nav-link">Nodejs</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/41f87d890d0a02af/" class="nav-link">博客搭建</a></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="更多" class="dropdown-title"><a href="/blog/more/" class="link-title">更多</a> <span class="title" style="display:none;">更多</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/pages/f2a556/" class="nav-link">学习</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/aea6571b7a8bae86/" class="nav-link">面试</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/2d615df9a36a98ed/" class="nav-link">心情杂货</a></li><li class="dropdown-item"><!----> <a href="/blog/pages/baaa02/" class="nav-link">实用技巧</a></li><li class="dropdown-item"><!----> <a href="/blog/friends/" class="nav-link">友情链接</a></li></ul></div></div><div class="nav-item"><a href="/blog/about/" class="nav-link">关于</a></div><div class="nav-item"><a href="/blog/pages/beb6c0bd8a66cea6/" class="nav-link">收藏</a></div><div class="nav-item"><div class="dropdown-wrapper"><button type="button" aria-label="索引" class="dropdown-title"><a href="/blog/archives/" class="link-title">索引</a> <span class="title" style="display:none;">索引</span> <span class="arrow right"></span></button> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/blog/categories/" class="nav-link">分类</a></li><li class="dropdown-item"><!----> <a href="/blog/tags/" class="nav-link">标签</a></li><li class="dropdown-item"><!----> <a href="/blog/archives/" class="nav-link">归档</a></li></ul></div></div> <a href="https://github.com/heBody/blog" target="_blank" rel="noopener noreferrer" class="repo-link">
    24. 

  24.     GitHub
    25. 

  25.     <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav>  <ul class="sidebar-links"><li><a href="/blog/pages/f344d070a1031ef7/" class="sidebar-link">ECMAScript 6 简介</a></li><li><a href="/blog/pages/c1edd70a6b7c7872/" class="sidebar-link">let 和 const 命令</a></li><li><a href="/blog/pages/b1ab10a62f7564da/" class="sidebar-link">变量的解构赋值</a></li><li><a href="/blog/pages/ca89eca8adeba5f4/" class="sidebar-link">字符串的扩展</a></li><li><a href="/blog/pages/a650b4a0ebfc9350/" class="sidebar-link">字符串的新增方法</a></li><li><a href="/blog/pages/0473261a6ab0ee8c/" class="sidebar-link">正则的扩展</a></li><li><a href="/blog/pages/5dfea9a0f2d1a392/" class="sidebar-link">数值的扩展</a></li><li><a href="/blog/pages/8ed309d668b20264/" class="sidebar-link">函数的扩展</a></li><li><a href="/blog/pages/e34009d60d8bc4b2/" class="sidebar-link">数组的扩展</a></li><li><a href="/blog/pages/b5e3e0a0ff6e9c25/" class="sidebar-link">对象的扩展</a></li><li><a href="/blog/pages/e85e68947502cf90/" class="sidebar-link">对象的新增方法</a></li><li><a href="/blog/pages/02c86eb2792f3262/" class="sidebar-link">Symbol</a></li><li><a href="/blog/pages/0c21dae358fca16b/" class="sidebar-link">Set 和 Map 数据结构</a></li><li><a href="/blog/pages/f56ec2ab97d60483/" class="sidebar-link">Proxy</a></li><li><a href="/blog/pages/74de3e45e4491e95/" class="sidebar-link">Reflect</a></li><li><a href="/blog/pages/2810ae8985e9bd52/" class="sidebar-link">Promise 对象</a></li><li><a href="/blog/pages/48df907ad3570f3d/" class="sidebar-link">Iterator 和 for-of 循环</a></li><li><a href="/blog/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/blog/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/blog/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/blog/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/blog/pages/83f8c3a0cd87dd83/" class="sidebar-link">Class 的继承</a></li><li><a href="/blog/pages/efe2fb04eb8ac5fb/" aria-current="page" class="active sidebar-link">Module 的语法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#概述" class="sidebar-link">概述</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#严格模式" class="sidebar-link">严格模式</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#export-命令" class="sidebar-link">export 命令</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#import-命令" class="sidebar-link">import 命令</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#模块的整体加载-星号" class="sidebar-link">模块的整体加载-(星号*)</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#export-default-命令" class="sidebar-link">export default 命令</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#export-与-import-的复合写法" class="sidebar-link">export 与 import 的复合写法</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#模块的继承" class="sidebar-link">模块的继承</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#跨模块常量" class="sidebar-link">跨模块常量</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/efe2fb04eb8ac5fb/#import" class="sidebar-link">import()</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/efe2fb04eb8ac5fb/#简介" class="sidebar-link">简介</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/efe2fb04eb8ac5fb/#适用场合" class="sidebar-link">适用场合</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/efe2fb04eb8ac5fb/#注意点" class="sidebar-link">注意点</a></li></ul></li></ul></li><li><a href="/blog/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/blog/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/blog/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/blog/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/blog/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">Module 的语法<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="module-的语法"><a href="#module-的语法" class="header-anchor">#</a> Module 的语法</h1> <h2 id="概述"><a href="#概述" class="header-anchor">#</a> 概述</h2> <p>历史上，JavaScript 一直没有模块（module）体系，无法将一个大程序拆分成互相依赖的小文件，再用简单的方法拼装起来。其他语言都有这项功能，比如 Ruby 的<code>require</code>、Python 的<code>import</code>，甚至就连 CSS 都有<code>@import</code>，但是 JavaScript 任何这方面的支持都没有，这对开发大型的、复杂的项目形成了巨大障碍。
    26. 

  26. 

  27. 在 ES6 之前，社区制定了一些模块加载方案，最主要的有 CommonJS 和 AMD 两种。前者用于服务器，后者用于浏览器。ES6 在语言标准的层面上，实现了模块功能，而且实现得相当简单，完全可以取代 CommonJS 和 AMD 规范，成为浏览器和服务器通用的模块解决方案。</p> <p>ES6 模块的设计思想是尽量的静态化，使得编译时就能确定模块的依赖关系，以及输入和输出的变量。CommonJS 和 AMD 模块，都只能在运行时确定这些东西。比如，CommonJS 模块就是对象，输入时必须查找对象属性。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// CommonJS模块</span>
    28. 

  28. <span class="token keyword">let</span> <span class="token punctuation">{</span> stat<span class="token punctuation">,</span> exists<span class="token punctuation">,</span> readFile <span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'fs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    29. 

  29. 

  30. <span class="token comment">// 等同于</span>
    31. 

  31. <span class="token keyword">let</span> _fs <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'fs'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    32. 

  32. <span class="token keyword">let</span> stat <span class="token operator">=</span> _fs<span class="token punctuation">.</span>stat<span class="token punctuation">;</span>
    33. 

  33. <span class="token keyword">let</span> exists <span class="token operator">=</span> _fs<span class="token punctuation">.</span>exists<span class="token punctuation">;</span>
    34. 

  34. <span class="token keyword">let</span> readfile <span class="token operator">=</span> _fs<span class="token punctuation">.</span>readfile<span class="token punctuation">;</span>
    35. 

  35. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码的实质是整体加载<code>fs</code>模块（即加载<code>fs</code>的所有方法），生成一个对象（<code>_fs</code>），然后再从这个对象上面读取 3 个方法。<strong>这种加载称为“运行时加载”</strong>，因为只有运行时才能得到这个对象，导致完全没办法在编译时做“静态优化”。</p> <p>ES6 模块不是对象，而是通过<code>export</code>命令显式指定输出的代码，再通过<code>import</code>命令输入。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES6模块</span>
    36. 

  36. <span class="token keyword">import</span> <span class="token punctuation">{</span> stat<span class="token punctuation">,</span> exists<span class="token punctuation">,</span> readFile <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'fs'</span><span class="token punctuation">;</span>
    37. 

  37. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码的实质是从<code>fs</code>模块加载 3 个方法，其他方法不加载。<strong>这种加载称为“编译时加载”或者静态加载</strong>，**即 ES6 可以在编译时就完成模块加载，**效率要比 CommonJS 模块的加载方式高。当然，这也导致了没法引用 ES6 模块本身，因为它不是对象。</p> <p>由于 ES6 模块是编译时加载，使得静态分析成为可能。有了它，就能进一步拓宽 JavaScript 的语法，比如引入宏（macro）和类型检验（type system）这些只能靠静态分析实现的功能。</p> <p>除了静态加载带来的各种好处，ES6 模块还有以下好处。</p> <ul><li>不再需要<code>UMD</code>模块格式了，将来服务器和浏览器都会支持 ES6 模块格式。目前，通过各种工具库，其实已经做到了这一点。</li> <li>将来浏览器的新 API 就能用模块格式提供，不再必须做成全局变量或者<code>navigator</code>对象的属性。</li> <li>不再需要对象作为命名空间（比如<code>Math</code>对象），未来这些功能可以通过模块提供。</li></ul> <p>本章介绍 ES6 模块的语法，下一章介绍如何在浏览器和 Node 之中，加载 ES6 模块。</p> <h2 id="严格模式"><a href="#严格模式" class="header-anchor">#</a> 严格模式</h2> <p><strong>ES6 的模块自动采用严格模式</strong>，不管你有没有在模块头部加上<code>&quot;use strict&quot;;</code>。</p> <p>严格模式主要有以下限制。</p> <ul><li>变量必须声明后再使用</li> <li>函数的参数不能有同名属性，否则报错</li> <li>不能使用<code>with</code>语句</li> <li>不能对只读属性赋值，否则报错</li> <li>不能使用前缀 0 表示八进制数，否则报错</li> <li>不能删除不可删除的属性，否则报错</li> <li>不能删除变量<code>delete prop</code>，会报错，只能删除属性<code>delete global[prop]</code></li> <li><code>eval</code>不会在它的外层作用域引入变量</li> <li><code>eval</code>和<code>arguments</code>不能被重新赋值</li> <li><code>arguments</code>不会自动反映函数参数的变化</li> <li>不能使用<code>arguments.callee</code></li> <li>不能使用<code>arguments.caller</code></li> <li>禁止<code>this</code>指向全局对象</li> <li>不能使用<code>fn.caller</code>和<code>fn.arguments</code>获取函数调用的堆栈</li> <li>增加了保留字（比如<code>protected</code>、<code>static</code>和<code>interface</code>）</li></ul> <p>上面这些限制，模块都必须遵守。由于严格模式是 ES5 引入的，不属于 ES6，所以请参阅相关 ES5 书籍，本书不再详细介绍了。</p> <p>其中，尤其需要注意<code>this</code>的限制。ES6 模块之中，顶层的<code>this</code>指向<code>undefined</code>，即不应该在顶层代码使用<code>this</code>。</p> <h2 id="export-命令"><a href="#export-命令" class="header-anchor">#</a> export 命令</h2> <p>模块功能主要由两个命令构成：<code>export</code>和<code>import</code>。<code>export</code>命令用于规定模块的对外接口，<code>import</code>命令用于输入其他模块提供的功能。</p> <p><strong>一个模块就是一个独立的文件。该文件内部的所有变量，外部无法获取。如果你希望外部能够读取模块内部的某个变量，就必须使用<code>export</code>关键字输出该变量</strong>。下面是一个 JS 文件，里面使用<code>export</code>命令输出变量。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// profile.js</span>
    38. 

  38. <span class="token keyword">export</span> <span class="token keyword">var</span> firstName <span class="token operator">=</span> <span class="token string">'Michael'</span><span class="token punctuation">;</span>
    39. 

  39. <span class="token keyword">export</span> <span class="token keyword">var</span> lastName <span class="token operator">=</span> <span class="token string">'Jackson'</span><span class="token punctuation">;</span>
    40. 

  40. <span class="token keyword">export</span> <span class="token keyword">var</span> year <span class="token operator">=</span> <span class="token number">1958</span><span class="token punctuation">;</span>
    41. 

  41. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码是<code>profile.js</code>文件，保存了用户信息。ES6 将其视为一个模块，里面用<code>export</code>命令对外部输出了三个变量。</p> <p><code>export</code>的写法，除了像上面这样，还有另外一种。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// profile.js</span>
    42. 

  42. <span class="token keyword">var</span> firstName <span class="token operator">=</span> <span class="token string">'Michael'</span><span class="token punctuation">;</span>
    43. 

  43. <span class="token keyword">var</span> lastName <span class="token operator">=</span> <span class="token string">'Jackson'</span><span class="token punctuation">;</span>
    44. 

  44. <span class="token keyword">var</span> year <span class="token operator">=</span> <span class="token number">1958</span><span class="token punctuation">;</span>
    45. 

  45. 

  46. <span class="token keyword">export</span> <span class="token punctuation">{</span> firstName<span class="token punctuation">,</span> lastName<span class="token punctuation">,</span> year <span class="token punctuation">}</span><span class="token punctuation">;</span>
    47. 

  47. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码在<code>export</code>命令后面，使用大括号指定所要输出的一组变量。它与前一种写法（直接放置在<code>var</code>语句前）是等价的，但是应该优先考虑使用这种写法。因为这样就可以在脚本尾部，一眼看清楚输出了哪些变量。</p> <p><code>export</code>命令除了输出变量，还可以输出函数或类（class）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">multiply</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    48. 

  48.   <span class="token keyword">return</span> x <span class="token operator">*</span> y<span class="token punctuation">;</span>
    49. 

  49. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    50. 

  50. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码对外输出一个函数<code>multiply</code>。</p> <p>通常情况下，<code>export</code>输出的变量就是本来的名字，但是<strong>可以使用<code>as</code>关键字重命名</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">v1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
    51. 

  51. <span class="token keyword">function</span> <span class="token function">v2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
    52. 

  52. 

  53. <span class="token keyword">export</span> <span class="token punctuation">{</span>
    54. 

  54.   v1 <span class="token keyword">as</span> streamV1<span class="token punctuation">,</span>
    55. 

  55.   v2 <span class="token keyword">as</span> streamV2<span class="token punctuation">,</span>
    56. 

  56.   v2 <span class="token keyword">as</span> streamLatestVersion
    57. 

  57. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    58. 

  58. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码使用<code>as</code>关键字，重命名了函数<code>v1</code>和<code>v2</code>的对外接口。重命名后，<code>v2</code>可以用不同的名字输出两次。</p> <p>需要特别注意的是，<code>export</code>命令规定的是对外的接口，必须与模块内部的变量建立一一对应关系。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    59. 

  59. <span class="token keyword">export</span> <span class="token number">1</span><span class="token punctuation">;</span>
    60. 

  60. 

  61. <span class="token comment">// 报错</span>
    62. 

  62. <span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    63. 

  63. <span class="token keyword">export</span> m<span class="token punctuation">;</span>
    64. 

  64. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面两种写法都会报错，因为没有提供对外的接口。第一种写法直接输出 1，第二种写法通过变量<code>m</code>，还是直接输出 1。<code>1</code>只是一个值，不是接口。正确的写法是下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 写法一</span>
    65. 

  65. <span class="token keyword">export</span> <span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    66. 

  66. 

  67. <span class="token comment">// 写法二</span>
    68. 

  68. <span class="token keyword">var</span> m <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    69. 

  69. <span class="token keyword">export</span> <span class="token punctuation">{</span>m<span class="token punctuation">}</span><span class="token punctuation">;</span>
    70. 

  70. 

  71. <span class="token comment">// 写法三</span>
    72. 

  72. <span class="token keyword">var</span> n <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    73. 

  73. <span class="token keyword">export</span> <span class="token punctuation">{</span>n <span class="token keyword">as</span> m<span class="token punctuation">}</span><span class="token punctuation">;</span>
    74. 

  74. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>上面三种写法都是正确的，规定了对外的接口<code>m</code>。其他脚本可以通过这个接口，取到值<code>1</code>。它们的实质是，在接口名与模块内部变量之间，建立了一一对应的关系。</p> <p>同样的，<code>function</code>和<code>class</code>的输出，也必须遵守这样的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    75. 

  75. <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    76. 

  76. <span class="token keyword">export</span> f<span class="token punctuation">;</span>
    77. 

  77. 

  78. <span class="token comment">// 正确</span>
    79. 

  79. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    80. 

  80. 

  81. <span class="token comment">// 正确</span>
    82. 

  82. <span class="token keyword">function</span> <span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    83. 

  83. <span class="token keyword">export</span> <span class="token punctuation">{</span>f<span class="token punctuation">}</span><span class="token punctuation">;</span>
    84. 

  84. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>另外，<code>export</code>语句输出的接口，与其对应的<strong>值是动态绑定关系</strong>，即通过该接口，可以取到模块内部实时的值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token string">'bar'</span><span class="token punctuation">;</span>
    85. 

  85. <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> foo <span class="token operator">=</span> <span class="token string">'baz'</span><span class="token punctuation">,</span> <span class="token number">500</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    86. 

  86. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码输出变量<code>foo</code>，值为<code>bar</code>，500 毫秒之后变成<code>baz</code>。</p> <p>这一点与 CommonJS 规范完全不同。CommonJS 模块输出的是值的缓存，不存在动态更新，详见下文《Module 的加载实现》一节。</p> <p>最后，<code>export</code>命令可以出现在模块的任何位置，只要处于模块顶层就可以。如果处于块级作用域内，就会报错，下一节的<code>import</code>命令也是如此。这是因为处于条件代码块之中，就没法做静态优化了，违背了 ES6 模块的设计初衷。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    87. 

  87.   <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token string">'bar'</span> <span class="token comment">// SyntaxError</span>
    88. 

  88. <span class="token punctuation">}</span>
    89. 

  89. <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    90. 

  90. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>export</code>语句放在函数之中，结果报错。</p> <h2 id="import-命令"><a href="#import-命令" class="header-anchor">#</a> import 命令</h2> <p>使用<code>export</code>命令定义了模块的对外接口以后，其他 JS 文件就可以通过<code>import</code>命令加载这个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// main.js</span>
    91. 

  91. <span class="token keyword">import</span> <span class="token punctuation">{</span> firstName<span class="token punctuation">,</span> lastName<span class="token punctuation">,</span> year <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./profile.js'</span><span class="token punctuation">;</span>
    92. 

  92. 

  93. <span class="token keyword">function</span> <span class="token function">setName</span><span class="token punctuation">(</span><span class="token parameter">element</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    94. 

  94.   element<span class="token punctuation">.</span>textContent <span class="token operator">=</span> firstName <span class="token operator">+</span> <span class="token string">' '</span> <span class="token operator">+</span> lastName<span class="token punctuation">;</span>
    95. 

  95. <span class="token punctuation">}</span>
    96. 

  96. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码的<code>import</code>命令，用于加载<code>profile.js</code>文件，并从中输入变量。<code>import</code>命令接受一对大括号，里面指定要从其他模块导入的变量名。大括号里面的<strong>变量名，必须与被导入模块（<code>profile.js</code>）对外接口的名称相同。</strong></p> <p>如果想为输入的变量重新取一个名字**，<code>import</code>命令要使用<code>as</code>关键字，将输入的变量重命名。**</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span> lastName <span class="token keyword">as</span> surname <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./profile.js'</span><span class="token punctuation">;</span>
    97. 

  97. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><strong><code>import</code>命令输入的变量都是只读的</strong>，因为它的本质是输入接口。也就是说，不允许在加载模块的脚本里面，改写接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span>a<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./xxx.js'</span>
    98. 

  98. 

  99. a <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// Syntax Error : 'a' is read-only;</span>
    100. 

  100. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，脚本加载了变量<code>a</code>，对其重新赋值就会报错，因为<code>a</code>是一个只读的接口。但是**，如果<code>a</code>是一个对象，改写<code>a</code>的属性是允许的**。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span>a<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./xxx.js'</span>
    101. 

  101. 

  102. a<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span> <span class="token comment">// 合法操作</span>
    103. 

  103. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，<code>a</code>的属性可以成功改写，并且其他模块也可以读到改写后的值。不过，这种写法很难查错，<strong>建议凡是输入的变量，都当作完全只读，不要轻易改变它的属性</strong>。</p> <p><code>import</code>后面的<code>from</code>指定模块文件的位置，可以是相对路径，也可以是绝对路径，<strong><code>.js</code>后缀可以省略</strong>。<strong>如果只是模块名，不带有路径，那么必须有配置文件</strong>，告诉 JavaScript 引擎该模块的位置。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span>myMethod<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'util'</span><span class="token punctuation">;</span>
    104. 

  104. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>util</code>是模块文件名，由于不带有路径，必须通过配置，告诉引擎怎么取到这个模块。</p> <p>注意，<strong><code>import</code>命令具有提升效果，会提升到整个模块的头部</strong>，首先执行。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    105. 

  105. 

  106. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    107. 

  107. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面的代码不会报错，因为<code>import</code>的执行早于<code>foo</code>的调用。这种行为的本质是，<code>import</code>命令是编译阶段执行的，在代码运行之前。</p> <p>由于<code>import</code>是静态执行，所以<strong>不能使用表达式和变量</strong>，这些只有在运行时才能得到结果的语法结构。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    108. 

  108. <span class="token keyword">import</span> <span class="token punctuation">{</span> <span class="token string">'f'</span> <span class="token operator">+</span> <span class="token string">'oo'</span> <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    109. 

  109. 

  110. <span class="token comment">// 报错</span>
    111. 

  111. <span class="token keyword">let</span> module <span class="token operator">=</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    112. 

  112. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> from module<span class="token punctuation">;</span>
    113. 

  113. 

  114. <span class="token comment">// 报错</span>
    115. 

  115. <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">===</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    116. 

  116.   <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'module1'</span><span class="token punctuation">;</span>
    117. 

  117. <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    118. 

  118.   <span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'module2'</span><span class="token punctuation">;</span>
    119. 

  119. <span class="token punctuation">}</span>
    120. 

  120. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面三种写法都会报错，因为它们用到了表达式、变量和<code>if</code>结构。在静态分析阶段，这些语法都是没法得到值的。</p> <p>最后，<code>import</code>语句会执行所加载的模块，因此可以有下面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    121. 

  121. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码仅仅执行<code>lodash</code>模块，但是不输入任何值。</p> <p>如果多次重复执行同一句<code>import</code>语句，那么只会执行一次，而不会执行多次。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    122. 

  122. <span class="token keyword">import</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    123. 

  123. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码加载了两次<code>lodash</code>，但是只会执行一次。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token punctuation">{</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    124. 

  124. <span class="token keyword">import</span> <span class="token punctuation">{</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    125. 

  125. 

  126. <span class="token comment">// 等同于</span>
    127. 

  127. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    128. 

  128. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，虽然<code>foo</code>和<code>bar</code>在两个语句中加载，但是它们对应的是同一个<code>my_module</code>实例。也就是说，<code>import</code>语句是 Singleton 模式。</p> <p>目前阶段，通过 Babel 转码，CommonJS 模块的<code>require</code>命令和 ES6 模块的<code>import</code>命令，可以写在同一个模块里面，但是最好不要这样做。因为<code>import</code>在静态解析阶段执行，所以它是一个模块之中最早执行的。下面的代码可能不会得到预期结果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'core-js/modules/es6.symbol'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    129. 

  129. <span class="token function">require</span><span class="token punctuation">(</span><span class="token string">'core-js/modules/es6.promise'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    130. 

  130. <span class="token keyword">import</span> React <span class="token keyword">from</span> <span class="token string">'React'</span><span class="token punctuation">;</span>
    131. 

  131. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="模块的整体加载-星号"><a href="#模块的整体加载-星号" class="header-anchor">#</a> 模块的整体加载-(星号*)</h2> <p>除了指定加载某个输出值，还可以使用整体加载，即用星号（<code>*</code>）指定一个对象，所有输出值都加载在这个对象上面。</p> <p>下面是一个<code>circle.js</code>文件，它输出两个方法<code>area</code>和<code>circumference</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// circle.js</span>
    132. 

  132. 

  133. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">area</span><span class="token punctuation">(</span><span class="token parameter">radius</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    134. 

  134.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">*</span> radius <span class="token operator">*</span> radius<span class="token punctuation">;</span>
    135. 

  135. <span class="token punctuation">}</span>
    136. 

  136. 

  137. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">circumference</span><span class="token punctuation">(</span><span class="token parameter">radius</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    138. 

  138.   <span class="token keyword">return</span> <span class="token number">2</span> <span class="token operator">*</span> Math<span class="token punctuation">.</span><span class="token constant">PI</span> <span class="token operator">*</span> radius<span class="token punctuation">;</span>
    139. 

  139. <span class="token punctuation">}</span>
    140. 

  140. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>现在，加载这个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// main.js</span>
    141. 

  141. 

  142. <span class="token keyword">import</span> <span class="token punctuation">{</span> area<span class="token punctuation">,</span> circumference <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./circle'</span><span class="token punctuation">;</span>
    143. 

  143. 

  144. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆面积：'</span> <span class="token operator">+</span> <span class="token function">area</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    145. 

  145. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆周长：'</span> <span class="token operator">+</span> <span class="token function">circumference</span><span class="token punctuation">(</span><span class="token number">14</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    146. 

  146. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面写法是逐一指定要加载的方法，整体加载的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> circle <span class="token keyword">from</span> <span class="token string">'./circle'</span><span class="token punctuation">;</span>
    147. 

  147. 

  148. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆面积：'</span> <span class="token operator">+</span> circle<span class="token punctuation">.</span><span class="token function">area</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    149. 

  149. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'圆周长：'</span> <span class="token operator">+</span> circle<span class="token punctuation">.</span><span class="token function">circumference</span><span class="token punctuation">(</span><span class="token number">14</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    150. 

  150. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>注意，<strong>模块整体加载所在的那个对象（上例是<code>circle</code>），应该是可以静态分析的，所以不允许运行时改变</strong>。下面的写法都是不允许的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> circle <span class="token keyword">from</span> <span class="token string">'./circle'</span><span class="token punctuation">;</span>
    151. 

  151. 

  152. <span class="token comment">// 下面两行都是不允许的</span>
    153. 

  153. circle<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">'hello'</span><span class="token punctuation">;</span>
    154. 

  154. circle<span class="token punctuation">.</span><span class="token function-variable function">area</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    155. 

  155. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h2 id="export-default-命令"><a href="#export-default-命令" class="header-anchor">#</a> export default 命令</h2> <p>从前面的例子可以看出，使用<code>import</code>命令的时候，用户需要知道所要加载的变量名或函数名，否则无法加载。但是，用户肯定希望快速上手，未必愿意阅读文档，去了解模块有哪些属性和方法。</p> <p>为了给用户提供方便，让他们不用阅读文档就能加载模块，就要用到**<code>export default</code>命令，为模块指定默认输出。**</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// export-default.js</span>
    156. 

  156. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    157. 

  157.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    158. 

  158. <span class="token punctuation">}</span>
    159. 

  159. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码是一个模块文件<code>export-default.js</code>，它的默认输出是一个函数。</p> <p>其他模块加载该模块时，<strong><code>import</code>命令可以为该匿名函数指定任意名字。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// import-default.js</span>
    160. 

  160. <span class="token keyword">import</span> customName <span class="token keyword">from</span> <span class="token string">'./export-default'</span><span class="token punctuation">;</span>
    161. 

  161. <span class="token function">customName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 'foo'</span>
    162. 

  162. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码的<code>import</code>命令，可以用任意名称指向<code>export-default.js</code>输出的方法，这时就不需要知道原模块输出的函数名。需要注意的是，<strong>这时<code>import</code>命令后面，不使用大括号</strong>。</p> <p><code>export default</code>命令用在非匿名函数前，也是可以的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// export-default.js</span>
    163. 

  163. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    164. 

  164.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    165. 

  165. <span class="token punctuation">}</span>
    166. 

  166. 

  167. <span class="token comment">// 或者写成</span>
    168. 

  168. 

  169. <span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    170. 

  170.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    171. 

  171. <span class="token punctuation">}</span>
    172. 

  172. 

  173. <span class="token keyword">export</span> <span class="token keyword">default</span> foo<span class="token punctuation">;</span>
    174. 

  174. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>上面代码中，<strong><code>foo</code>函数的函数名<code>foo</code>，在模块外部是无效的。加载的时候，视同匿名函数加载</strong>。</p> <p>下面比较一下默认输出和正常输出。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 第一组</span>
    175. 

  175. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span> <span class="token function">crc32</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 输出</span>
    176. 

  176.   <span class="token comment">// ...</span>
    177. 

  177. <span class="token punctuation">}</span>
    178. 

  178. 

  179. <span class="token keyword">import</span> crc32 <span class="token keyword">from</span> <span class="token string">'crc32'</span><span class="token punctuation">;</span> <span class="token comment">// 输入</span>
    180. 

  180. 

  181. <span class="token comment">// 第二组</span>
    182. 

  182. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">crc32</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 输出</span>
    183. 

  183.   <span class="token comment">// ...</span>
    184. 

  184. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    185. 

  185. 

  186. <span class="token keyword">import</span> <span class="token punctuation">{</span>crc32<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'crc32'</span><span class="token punctuation">;</span> <span class="token comment">// 输入</span>
    187. 

  187. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>上面代码的两组写法，第一组是使用<code>export default</code>时，对应的<code>import</code>语句不需要使用大括号；第二组是不使用<code>export default</code>时，对应的<code>import</code>语句需要使用大括号。</p> <p><code>export default</code>命令用于指定模块的默认输出。显然，<strong>一个模块只能有一个默认输出</strong>，因此<code>export default</code>命令只能使用一次。所以，import命令后面才不用加大括号，因为只可能唯一对应<code>export default</code>命令。</p> <p>**本质上，<code>export default</code>就是输出一个叫做<code>default</code>的变量或方法，**然后系统允许你为它取任意名字。所以，下面的写法是有效的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// modules.js</span>
    188. 

  188. <span class="token keyword">function</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span> y</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    189. 

  189.   <span class="token keyword">return</span> x <span class="token operator">*</span> y<span class="token punctuation">;</span>
    190. 

  190. <span class="token punctuation">}</span>
    191. 

  191. <span class="token keyword">export</span> <span class="token punctuation">{</span>add <span class="token keyword">as</span> <span class="token keyword">default</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    192. 

  192. <span class="token comment">// 等同于</span>
    193. 

  193. <span class="token comment">// export default add;</span>
    194. 

  194. 

  195. <span class="token comment">// app.js</span>
    196. 

  196. <span class="token keyword">import</span> <span class="token punctuation">{</span> <span class="token keyword">default</span> <span class="token keyword">as</span> foo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'modules'</span><span class="token punctuation">;</span>
    197. 

  197. <span class="token comment">// 等同于</span>
    198. 

  198. <span class="token comment">// import foo from 'modules';</span>
    199. 

  199. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>正是因为<code>export default</code>命令其实只是输出一个叫做<code>default</code>的变量，所以它后面不能跟变量声明语句。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 正确</span>
    200. 

  200. <span class="token keyword">export</span> <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    201. 

  201. 

  202. <span class="token comment">// 正确</span>
    203. 

  203. <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    204. 

  204. <span class="token keyword">export</span> <span class="token keyword">default</span> a<span class="token punctuation">;</span>
    205. 

  205. 

  206. <span class="token comment">// 错误</span>
    207. 

  207. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    208. 

  208. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>export default a</code>的含义是将变量<code>a</code>的值赋给变量<code>default</code>。所以，最后一种写法会报错。</p> <p>同样地，因为<code>export default</code>命令的本质是将后面的值，赋给<code>default</code>变量，所以可以直接将一个值写在<code>export default</code>之后。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 正确</span>
    209. 

  209. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token number">42</span><span class="token punctuation">;</span>
    210. 

  210. 

  211. <span class="token comment">// 报错</span>
    212. 

  212. <span class="token keyword">export</span> <span class="token number">42</span><span class="token punctuation">;</span>
    213. 

  213. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，后一句报错是因为没有指定对外的接口，而前一句指定对外接口为<code>default</code>。</p> <p>有了<code>export default</code>命令，输入模块时就非常直观了，以输入 lodash 模块为例。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> _ <span class="token keyword">from</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    214. 

  214. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>如果想在一条<code>import</code>语句中，同时输入默认方法和其他接口，可以写成下面这样。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> _<span class="token punctuation">,</span> <span class="token punctuation">{</span> each<span class="token punctuation">,</span> forEach <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'lodash'</span><span class="token punctuation">;</span>
    215. 

  215. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>对应上面代码的<code>export</code>语句如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    216. 

  216.   <span class="token comment">// ···</span>
    217. 

  217. <span class="token punctuation">}</span>
    218. 

  218. 

  219. <span class="token keyword">export</span> <span class="token keyword">function</span> <span class="token function">each</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> iterator<span class="token punctuation">,</span> context</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    220. 

  220.   <span class="token comment">// ···</span>
    221. 

  221. <span class="token punctuation">}</span>
    222. 

  222. 

  223. <span class="token keyword">export</span> <span class="token punctuation">{</span> each <span class="token keyword">as</span> forEach <span class="token punctuation">}</span><span class="token punctuation">;</span>
    224. 

  224. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码的最后一行的意思是，暴露出<code>forEach</code>接口，默认指向<code>each</code>接口，即<code>forEach</code>和<code>each</code>指向同一个方法。</p> <p><code>export default</code>也可以用来输出类。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// MyClass.js</span>
    225. 

  225. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">class</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span>
    226. 

  226. 

  227. <span class="token comment">// main.js</span>
    228. 

  228. <span class="token keyword">import</span> MyClass <span class="token keyword">from</span> <span class="token string">'MyClass'</span><span class="token punctuation">;</span>
    229. 

  229. <span class="token keyword">let</span> o <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">MyClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    230. 

  230. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h2 id="export-与-import-的复合写法"><a href="#export-与-import-的复合写法" class="header-anchor">#</a> export 与 import 的复合写法</h2> <p>如果在一个模块之中，先输入后输出同一个模块，<code>import</code>语句可以与<code>export</code>语句写在一起。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    231. 

  231. 

  232. <span class="token comment">// 可以简单理解为</span>
    233. 

  233. <span class="token keyword">import</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    234. 

  234. <span class="token keyword">export</span> <span class="token punctuation">{</span> foo<span class="token punctuation">,</span> bar <span class="token punctuation">}</span><span class="token punctuation">;</span>
    235. 

  235. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>export</code>和<code>import</code>语句可以结合在一起，写成一行。但需要注意的是，写成一行以后，<code>foo</code>和<code>bar</code>实际上并没有被导入当前模块，只是相当于对外转发了这两个接口，导致当前模块不能直接使用<code>foo</code>和<code>bar</code>。</p> <p>模块的接口改名和整体输出，也可以采用这种写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 接口改名</span>
    236. 

  236. <span class="token keyword">export</span> <span class="token punctuation">{</span> foo <span class="token keyword">as</span> myFoo <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    237. 

  237. 

  238. <span class="token comment">// 整体输出</span>
    239. 

  239. <span class="token keyword">export</span> <span class="token operator">*</span> <span class="token keyword">from</span> <span class="token string">'my_module'</span><span class="token punctuation">;</span>
    240. 

  240. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>默认接口的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> <span class="token keyword">default</span> <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'foo'</span><span class="token punctuation">;</span>
    241. 

  241. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>具名接口改为默认接口的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> es6 <span class="token keyword">as</span> <span class="token keyword">default</span> <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./someModule'</span><span class="token punctuation">;</span>
    242. 

  242. 

  243. <span class="token comment">// 等同于</span>
    244. 

  244. <span class="token keyword">import</span> <span class="token punctuation">{</span> es6 <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./someModule'</span><span class="token punctuation">;</span>
    245. 

  245. <span class="token keyword">export</span> <span class="token keyword">default</span> es6<span class="token punctuation">;</span>
    246. 

  246. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>同样地，默认接口也可以改名为具名接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token punctuation">{</span> <span class="token keyword">default</span> <span class="token keyword">as</span> es6 <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./someModule'</span><span class="token punctuation">;</span>
    247. 

  247. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>下面三种<code>import</code>语句，没有对应的复合写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    248. 

  248. <span class="token keyword">import</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    249. 

  249. <span class="token keyword">import</span> someIdentifier<span class="token punctuation">,</span> <span class="token punctuation">{</span> namedIdentifier <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    250. 

  250. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>为了做到形式的对称，现在有<a href="https://github.com/leebyron/ecmascript-export-default-from" target="_blank" rel="noopener noreferrer">提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>，提出补上这三种复合写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">export</span> <span class="token operator">*</span> <span class="token keyword">as</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    251. 

  251. <span class="token keyword">export</span> someIdentifier <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    252. 

  252. <span class="token keyword">export</span> someIdentifier<span class="token punctuation">,</span> <span class="token punctuation">{</span> namedIdentifier <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">&quot;someModule&quot;</span><span class="token punctuation">;</span>
    253. 

  253. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h2 id="模块的继承"><a href="#模块的继承" class="header-anchor">#</a> 模块的继承</h2> <p>模块之间也可以继承。</p> <p>假设有一个<code>circleplus</code>模块，继承了<code>circle</code>模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// circleplus.js</span>
    254. 

  254. 

  255. <span class="token keyword">export</span> <span class="token operator">*</span> <span class="token keyword">from</span> <span class="token string">'circle'</span><span class="token punctuation">;</span>
    256. 

  256. <span class="token keyword">export</span> <span class="token keyword">var</span> e <span class="token operator">=</span> <span class="token number">2.71828182846</span><span class="token punctuation">;</span>
    257. 

  257. <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    258. 

  258.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">exp</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    259. 

  259. <span class="token punctuation">}</span>
    260. 

  260. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中的<code>export *</code>，表示再输出<code>circle</code>模块的所有属性和方法。注意，<code>export *</code>命令会忽略<code>circle</code>模块的<code>default</code>方法。然后，上面代码又输出了自定义的<code>e</code>变量和默认方法。</p> <p>这时，也可以将<code>circle</code>的属性或方法，改名后再输出。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// circleplus.js</span>
    261. 

  261. 

  262. <span class="token keyword">export</span> <span class="token punctuation">{</span> area <span class="token keyword">as</span> circleArea <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'circle'</span><span class="token punctuation">;</span>
    263. 

  263. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码表示，只输出<code>circle</code>模块的<code>area</code>方法，且将其改名为<code>circleArea</code>。</p> <p>加载上面模块的写法如下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// main.js</span>
    264. 

  264. 

  265. <span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> math <span class="token keyword">from</span> <span class="token string">'circleplus'</span><span class="token punctuation">;</span>
    266. 

  266. <span class="token keyword">import</span> exp <span class="token keyword">from</span> <span class="token string">'circleplus'</span><span class="token punctuation">;</span>
    267. 

  267. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">exp</span><span class="token punctuation">(</span>math<span class="token punctuation">.</span>e<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    268. 

  268. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中的<code>import exp</code>表示，将<code>circleplus</code>模块的默认方法加载为<code>exp</code>方法。</p> <h2 id="跨模块常量"><a href="#跨模块常量" class="header-anchor">#</a> 跨模块常量</h2> <p>本书介绍<code>const</code>命令的时候说过，<code>const</code>声明的常量只在当前代码块有效。如果想设置跨模块的常量（即跨多个文件），或者说一个值要被多个模块共享，可以采用下面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// constants.js 模块</span>
    269. 

  269. <span class="token keyword">export</span> <span class="token keyword">const</span> <span class="token constant">A</span> <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    270. 

  270. <span class="token keyword">export</span> <span class="token keyword">const</span> <span class="token constant">B</span> <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    271. 

  271. <span class="token keyword">export</span> <span class="token keyword">const</span> <span class="token constant">C</span> <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span>
    272. 

  272. 

  273. <span class="token comment">// test1.js 模块</span>
    274. 

  274. <span class="token keyword">import</span> <span class="token operator">*</span> <span class="token keyword">as</span> constants <span class="token keyword">from</span> <span class="token string">'./constants'</span><span class="token punctuation">;</span>
    275. 

  275. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>constants<span class="token punctuation">.</span><span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1</span>
    276. 

  276. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>constants<span class="token punctuation">.</span><span class="token constant">B</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
    277. 

  277. 

  278. <span class="token comment">// test2.js 模块</span>
    279. 

  279. <span class="token keyword">import</span> <span class="token punctuation">{</span><span class="token constant">A</span><span class="token punctuation">,</span> <span class="token constant">B</span><span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./constants'</span><span class="token punctuation">;</span>
    280. 

  280. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token constant">A</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1</span>
    281. 

  281. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token constant">B</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 3</span>
    282. 

  282. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>如果要使用的常量非常多，可以建一个专门的<code>constants</code>目录，将各种常量写在不同的文件里面，保存在该目录下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// constants/db.js</span>
    283. 

  283. <span class="token keyword">export</span> <span class="token keyword">const</span> db <span class="token operator">=</span> <span class="token punctuation">{</span>
    284. 

  284.   <span class="token literal-property property">url</span><span class="token operator">:</span> <span class="token string">'http://my.couchdbserver.local:5984'</span><span class="token punctuation">,</span>
    285. 

  285.   <span class="token literal-property property">admin_username</span><span class="token operator">:</span> <span class="token string">'admin'</span><span class="token punctuation">,</span>
    286. 

  286.   <span class="token literal-property property">admin_password</span><span class="token operator">:</span> <span class="token string">'admin password'</span>
    287. 

  287. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    288. 

  288. 

  289. <span class="token comment">// constants/user.js</span>
    290. 

  290. <span class="token keyword">export</span> <span class="token keyword">const</span> users <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token string">'root'</span><span class="token punctuation">,</span> <span class="token string">'admin'</span><span class="token punctuation">,</span> <span class="token string">'staff'</span><span class="token punctuation">,</span> <span class="token string">'ceo'</span><span class="token punctuation">,</span> <span class="token string">'chief'</span><span class="token punctuation">,</span> <span class="token string">'moderator'</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    291. 

  291. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>然后，将这些文件输出的常量，合并在<code>index.js</code>里面。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// constants/index.js</span>
    292. 

  292. <span class="token keyword">export</span> <span class="token punctuation">{</span>db<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./db'</span><span class="token punctuation">;</span>
    293. 

  293. <span class="token keyword">export</span> <span class="token punctuation">{</span>users<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./users'</span><span class="token punctuation">;</span>
    294. 

  294. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>使用的时候，直接加载<code>index.js</code>就可以了。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// script.js</span>
    295. 

  295. <span class="token keyword">import</span> <span class="token punctuation">{</span>db<span class="token punctuation">,</span> users<span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./constants/index'</span><span class="token punctuation">;</span>
    296. 

  296. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="import"><a href="#import" class="header-anchor">#</a> import()</h2> <h3 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h3> <p>前面介绍过，<code>import</code>命令会被 JavaScript 引擎静态分析，先于模块内的其他语句执行（<code>import</code>命令叫做“连接” binding 其实更合适）。所以，下面的代码会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 报错</span>
    297. 

  297. <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">===</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    298. 

  298.   <span class="token keyword">import</span> MyModual <span class="token keyword">from</span> <span class="token string">'./myModual'</span><span class="token punctuation">;</span>
    299. 

  299. <span class="token punctuation">}</span>
    300. 

  300. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，引擎处理<code>import</code>语句是在编译时，这时不会去分析或执行<code>if</code>语句，所以<code>import</code>语句放在<code>if</code>代码块之中毫无意义，因此会报句法错误，而不是执行时错误。也就是说，<code>import</code>和<code>export</code>命令只能在模块的顶层，不能在代码块之中（比如，在<code>if</code>代码块之中，或在函数之中）。</p> <p>这样的设计，固然有利于编译器提高效率，但也导致无法在运行时加载模块。在语法上，条件加载就不可能实现。如果<code>import</code>命令要取代 Node 的<code>require</code>方法，这就形成了一个障碍。因为<code>require</code>是运行时加载模块，<code>import</code>命令无法取代<code>require</code>的动态加载功能。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> path <span class="token operator">=</span> <span class="token string">'./'</span> <span class="token operator">+</span> fileName<span class="token punctuation">;</span>
    301. 

  301. <span class="token keyword">const</span> myModual <span class="token operator">=</span> <span class="token function">require</span><span class="token punctuation">(</span>path<span class="token punctuation">)</span><span class="token punctuation">;</span>
    302. 

  302. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面的语句就是动态加载，<code>require</code>到底加载哪一个模块，只有运行时才知道。<code>import</code>命令做不到这一点。</p> <p><strong><a href="https://github.com/tc39/proposal-dynamic-import" target="_blank" rel="noopener noreferrer">ES2020提案<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 引入<code>import()</code>函数，支持动态加载模块。</strong></p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span>specifier<span class="token punctuation">)</span>
    303. 

  303. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>import</code>函数的参数<code>specifier</code>，指定所要加载的模块的位置。<code>import</code>命令能够接受什么参数，<code>import()</code>函数就能接受什么参数，两者区别主要是后者为动态加载。</p> <p><strong><code>import()</code>返回一个 Promise 对象</strong>。下面是一个例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> main <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelector</span><span class="token punctuation">(</span><span class="token string">'main'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    304. 

  304. 

  305. <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">./section-modules/</span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>someVariable<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string">.js</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">)</span>
    306. 

  306.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">module</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    307. 

  307.     module<span class="token punctuation">.</span><span class="token function">loadPageInto</span><span class="token punctuation">(</span>main<span class="token punctuation">)</span><span class="token punctuation">;</span>
    308. 

  308.   <span class="token punctuation">}</span><span class="token punctuation">)</span>
    309. 

  309.   <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">err</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    310. 

  310.     main<span class="token punctuation">.</span>textContent <span class="token operator">=</span> err<span class="token punctuation">.</span>message<span class="token punctuation">;</span>
    311. 

  311.   <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    312. 

  312. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p><strong><code>import()</code>函数可以用在任何地方，不仅仅是模块，非模块的脚本也可以使用</strong>。它是运行时执行，也就是说，什么时候运行到这一句，就会加载指定的模块。另外，<code>import()</code>函数与所加载的模块没有静态连接关系，这点也是与<code>import</code>语句不相同。<code>import()</code>类似于 Node 的<code>require</code>方法，区别主要是前者是异步加载，后者是同步加载。</p> <h3 id="适用场合"><a href="#适用场合" class="header-anchor">#</a> 适用场合</h3> <p>下面是<code>import()</code>的一些适用场合。</p> <p>（1）按需加载。</p> <p><code>import()</code>可以在需要的时候，再加载某个模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>button<span class="token punctuation">.</span><span class="token function">addEventListener</span><span class="token punctuation">(</span><span class="token string">'click'</span><span class="token punctuation">,</span> <span class="token parameter">event</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    313. 

  313.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./dialogBox.js'</span><span class="token punctuation">)</span>
    314. 

  314.   <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">dialogBox</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    315. 

  315.     dialogBox<span class="token punctuation">.</span><span class="token function">open</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    316. 

  316.   <span class="token punctuation">}</span><span class="token punctuation">)</span>
    317. 

  317.   <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token parameter">error</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    318. 

  318.     <span class="token comment">/* Error handling */</span>
    319. 

  319.   <span class="token punctuation">}</span><span class="token punctuation">)</span>
    320. 

  320. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    321. 

  321. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>import()</code>方法放在<code>click</code>事件的监听函数之中，只有用户点击了按钮，才会加载这个模块。</p> <p>（2）条件加载</p> <p><code>import()</code>可以放在<code>if</code>代码块，根据不同的情况，加载不同的模块。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span>condition<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    322. 

  322.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'moduleA'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    323. 

  323. <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    324. 

  324.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'moduleB'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    325. 

  325. <span class="token punctuation">}</span>
    326. 

  326. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，如果满足条件，就加载模块 A，否则加载模块 B。</p> <p>（3）动态的模块路径</p> <p><code>import()</code>允许模块路径动态生成。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token function">f</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    327. 

  327. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token operator">...</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    328. 

  328. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，根据函数<code>f</code>的返回结果，加载不同的模块。</p> <h3 id="注意点"><a href="#注意点" class="header-anchor">#</a> 注意点</h3> <p><code>import()</code>加载模块成功以后，这个模块会作为一个对象，当作<code>then</code>方法的参数。因此，可以使用对象解构赋值的语法，获取输出接口。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span>
    329. 

  329. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span>export1<span class="token punctuation">,</span> export2<span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    330. 

  330.   <span class="token comment">// ...·</span>
    331. 

  331. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    332. 

  332. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>export1</code>和<code>export2</code>都是<code>myModule.js</code>的输出接口，可以解构获得。</p> <p>如果模块有<code>default</code>输出接口，可以用参数直接获得。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span>
    333. 

  333. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">myModule</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    334. 

  334.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>myModule<span class="token punctuation">.</span>default<span class="token punctuation">)</span><span class="token punctuation">;</span>
    335. 

  335. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    336. 

  336. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面的代码也可以使用具名输入的形式。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span>
    337. 

  337. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">{</span><span class="token keyword">default</span><span class="token operator">:</span> theDefault<span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    338. 

  338.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>theDefault<span class="token punctuation">)</span><span class="token punctuation">;</span>
    339. 

  339. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    340. 

  340. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>如果想同时加载多个模块，可以采用下面的写法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
    341. 

  341.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module1.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    342. 

  342.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module2.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    343. 

  343.   <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module3.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    344. 

  344. <span class="token punctuation">]</span><span class="token punctuation">)</span>
    345. 

  345. <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter"><span class="token punctuation">[</span>module1<span class="token punctuation">,</span> module2<span class="token punctuation">,</span> module3<span class="token punctuation">]</span></span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    346. 

  346.    ···
    347. 

  347. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    348. 

  348. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p><code>import()</code>也可以用在 async 函数之中。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">async</span> <span class="token keyword">function</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    349. 

  349.   <span class="token keyword">const</span> myModule <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    350. 

  350.   <span class="token keyword">const</span> <span class="token punctuation">{</span>export1<span class="token punctuation">,</span> export2<span class="token punctuation">}</span> <span class="token operator">=</span> <span class="token keyword">await</span> <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./myModule.js'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    351. 

  351.   <span class="token keyword">const</span> <span class="token punctuation">[</span>module1<span class="token punctuation">,</span> module2<span class="token punctuation">,</span> module3<span class="token punctuation">]</span> <span class="token operator">=</span>
    352. 

  352.     <span class="token keyword">await</span> Promise<span class="token punctuation">.</span><span class="token function">all</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
    353. 

  353.       <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module1.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    354. 

  354.       <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module2.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    355. 

  355.       <span class="token keyword">import</span><span class="token punctuation">(</span><span class="token string">'./module3.js'</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    356. 

  356.     <span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    357. 

  357. <span class="token punctuation">}</span>
    358. 

  358. <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    359. 

  359. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《ES6 教程》笔记/23.Module 的语法.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=ES6" title="标签">#ES6</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/83f8c3a0cd87dd83/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">Class 的继承</div></a> <a href="/blog/pages/a79ca2e64ceae213/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">Module 的加载实现</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
    360. 

  360.         ←
    361. 

  361.         <a href="/blog/pages/83f8c3a0cd87dd83/" class="prev">Class 的继承</a></span> <span class="next"><a href="/blog/pages/a79ca2e64ceae213/">Module 的加载实现</a>→
    362. 

  362.       </span></p></div></div></div> <div class="article-list"><div class="article-title"><a href="/blog/archives/" class="iconfont icon-bi">最近更新</a></div> <div class="article-wrapper"><dl><dd>01</dd> <dt><a href="/blog/pages/922650/"><div>
    363. 

  363.             Git修改分支名
    364. 

  364.             <!----></div></a> <span class="date">08-11</span></dt></dl><dl><dd>02</dd> <dt><a href="/blog/pages/55f894/"><div>
    365. 

  365.             CSS给table的tbody添加滚动条
    366. 

  366.             <!----></div></a> <span class="date">06-29</span></dt></dl><dl><dd>03</dd> <dt><a href="/blog/pages/829589/"><div>
    367. 

  367.             我做了一个手写春联小网页，祝大家虎年暴富
    368. 

  368.             <span class="title-tag">
    369. 

  369.               原创
    370. 

  370.             </span></div></a> <span class="date">01-28</span></dt></dl> <dl><dd></dd> <dt><a href="/blog/archives/" class="more">更多文章&gt;</a></dt></dl></div></div></main></div> <div class="footer"><div class="icons"><a href="mailto:30363811@qq.com" title="发邮件" target="_blank" class="iconfont icon-youjian"></a><a href="https://github.com/heBody" title="GitHub" target="_blank" class="iconfont icon-github"></a></div> 
    371. 

  371.     Copyright © 2016-2022
    372. 

  372.     <span>Hesb | <a href="https://github.com/heBody/blob" target="_blank">MIT License</a></span></div> <div class="buttons"><div title="返回顶部" class="button blur go-to-top iconfont icon-fanhuidingbu" style="display:none;"></div> <div title="去评论" class="button blur go-to-comment iconfont icon-pinglun" style="display:none;"></div> <div title="主题模式" class="button blur theme-mode-but iconfont icon-zhuti"><ul class="select-box" style="display:none;"><li class="iconfont icon-zidong">
    373. 

  373.           跟随系统
    374. 

  374.         </li><li class="iconfont icon-rijianmoshi">
    375. 

  375.           浅色模式
    376. 

  376.         </li><li class="iconfont icon-yejianmoshi">
    377. 

  377.           深色模式
    378. 

  378.         </li><li class="iconfont icon-yuedu">
    379. 

  379.           阅读模式
    380. 

  380.         </li></ul></div></div> <!----> <!----> <!----></div><div class="global-ui"><div></div></div></div>
    381. 

  381.     <script src="/blog/assets/js/app.68c9df80.js" defer></script><script src="/blog/assets/js/2.395c0d18.js" defer></script><script src="/blog/assets/js/3.6748bd5c.js" defer></script><script src="/blog/assets/js/110.f41596b0.js" defer></script>
    382. 

  382.   </body>
    383. 

  383. </html>
    384.