(window.webpackJsonp=window.webpackJsonp||[]).push([[111],{441:function(s,t,a){"use strict";a.r(t);var n=a(3),e=Object(n.a)({},(function(){var s=this,t=s._self._c;return t("ContentSlotsDistributor",{attrs:{"slot-key":s.$parent.slotKey}},[t("h1",{attrs:{id:"module-的加载实现"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#module-的加载实现"}},[s._v("#")]),s._v(" Module 的加载实现")]),s._v(" "),t("p",[s._v("上一章介绍了模块的语法，本章介绍如何在浏览器和 Node.js 之中加载 ES6 模块，以及实际开发中经常遇到的一些问题（比如循环加载）。\n")]),s._v(" "),t("h2",{attrs:{id:"浏览器加载"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#浏览器加载"}},[s._v("#")]),s._v(" 浏览器加载")]),s._v(" "),t("h3",{attrs:{id:"传统方法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#传统方法"}},[s._v("#")]),s._v(" 传统方法")]),s._v(" "),t("p",[s._v("HTML 网页中，浏览器通过"),t("code",[s._v("<script>")]),s._v("标签加载 JavaScript 脚本。")]),s._v(" "),t("div",{staticClass:"language-html line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-html"}},[t("code",[t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br")])]),t("p",[s._v("上面代码中，老版本的 Node.js （不支持 ES6 模块）的入口文件是"),t("code",[s._v("main-legacy.cjs")]),s._v("，新版本的 Node.js 的入口文件是"),t("code",[s._v("main-modern.cjs")]),s._v("。")]),s._v(" "),t("p",[t("strong",[s._v("（3）条件加载")])]),s._v(" "),t("p",[s._v("利用"),t("code",[s._v(".")]),s._v("这个别名，可以为 ES6 模块和 CommonJS 指定不同的入口。目前，这个功能需要在 Node.js 运行的时候，打开"),t("code",[s._v("--experimental-conditional-exports")]),s._v("标志。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br")])]),t("p",[s._v("上面的代码证明了两件事。一是，在"),t("code",[s._v("b.js")]),s._v("之中，"),t("code",[s._v("a.js")]),s._v("没有执行完毕，只执行了第一行。二是，"),t("code",[s._v("main.js")]),s._v("执行到第二行时，不会再次执行"),t("code",[s._v("b.js")]),s._v("，而是输出缓存的"),t("code",[s._v("b.js")]),s._v("的执行结果，即它的第四行。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("exports"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("done "),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("上面代码中，如果发生循环加载，"),t("code",[s._v("require('a').foo")]),s._v("的值很可能后面会被改写，改用"),t("code",[s._v("require('a')")]),s._v("会更保险一点。")]),s._v(" "),t("h3",{attrs:{id:"es6-模块的循环加载"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#es6-模块的循环加载"}},[s._v("#")]),s._v(" ES6 模块的循环加载")]),s._v(" "),t("p",[s._v("ES6 处理“循环加载”与 CommonJS 有本质的不同。ES6 模块是动态引用，如果使用"),t("code",[s._v("import")]),s._v("从一个模块加载变量（即"),t("code",[s._v("import foo from 'foo'")]),s._v("），那些变量不会被缓存，而是成为一个指向被加载模块的引用，需要开发者自己保证，真正取值的时候能够取到值。")]),s._v(" "),t("p",[s._v("请看下面这个例子。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// a.mjs")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("import")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("bar"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("from")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'./b'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br")])]),t("p",[s._v("上面代码中，"),t("code",[s._v("a.mjs")]),s._v("加载"),t("code",[s._v("b.mjs")]),s._v("，"),t("code",[s._v("b.mjs")]),s._v("又加载"),t("code",[s._v("a.mjs")]),s._v("，构成循环加载。执行"),t("code",[s._v("a.mjs")]),s._v("，结果如下。")]),s._v(" "),t("div",{staticClass:"language-bash 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