(window.webpackJsonp=window.webpackJsonp||[]).push([[149],{478:function(t,s,a){"use strict";a.r(s);var n=a(3),e=Object(n.a)({},(function(){var t=this,s=t._self._c;return s("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[s("h1",{attrs:{id:"_07-列表-key"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_07-列表-key"}},[t._v("#")]),t._v(" 07. 列表 & Key")]),t._v(" "),s("p",[t._v("如下代码，我们使用 "),s("a",{attrs:{href:"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map",target:"_blank",rel:"noopener noreferrer"}},[s("code",[t._v("map()")]),s("OutboundLink")],1),t._v(" 函数让数组中的每一项变双倍，然后我们得到了一个新的列表 "),s("code",[t._v("doubled")]),t._v(" 并打印出来：")]),t._v(" "),s("div",{staticClass:"language-js line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-js"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" numbers "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" 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key")]),t._v(" "),s("p",[s("strong",[t._v("key 帮助 React 识别哪些元素改变了")]),t._v("，比如被添加或删除。因此你应当给数组中的每一个元素赋予一个确定的标识。")]),t._v(" "),s("p",[t._v("一个元素的 key 最好是这个元素在列表中拥有的"),s("strong",[t._v("一个独一无二的字符串")]),t._v("。"),s("strong",[t._v("通常，我们使用数据中的 id 来作为元素的 key")])]),t._v(" "),s("div",{staticClass:"language-jsx line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-jsx"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" todoItems "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" todos"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("map")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("todo")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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"),s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token tag"}},[s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("<")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ListItem")])]),t._v(" "),s("span",{pre:!0,attrs:{class:"token attr-name"}},[t._v("key")]),s("span",{pre:!0,attrs:{class:"token script language-javascript"}},[s("span",{pre:!0,attrs:{class:"token script-punctuation punctuation"}},[t._v("=")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("number"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("toString")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")])]),t._v(" "),s("span",{pre:!0,attrs:{class:"token attr-name"}},[t._v("value")]),s("span",{pre:!0,attrs:{class:"token script 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嵌套了太多层级，那可能就是你"),s("a",{attrs:{href:"https://zh-hans.reactjs.org/docs/components-and-props.html#extracting-components",target:"_blank",rel:"noopener noreferrer"}},[t._v("提取组件"),s("OutboundLink")],1),t._v("的一个好时机。")])])}),[],!1,null,null,null);s.default=e.exports}}]);