{"id":2234,"date":"2017-05-05T16:01:20","date_gmt":"2017-05-05T08:01:20","guid":{"rendered":"http:\/\/www.qdabc.cn\/?p=2234"},"modified":"2017-05-05T16:01:20","modified_gmt":"2017-05-05T08:01:20","slug":"es6%e4%b9%8blet%e8%83%bd%e6%9b%bf%e4%bb%a3var%e5%90%97","status":"publish","type":"post","link":"http:\/\/www.qdabc.cn\/?p=2234","title":{"rendered":"ES6\u4e4b&#8221;let&#8221;\u80fd\u66ff\u4ee3&#8221;var&#8221;\u5417?"},"content":{"rendered":"<p><strong>\u4e3a\u4e86\u4fdd\u8bc1\u53ef\u8bfb\u6027\uff0c\u672c\u6587\u91c7\u7528\u610f\u8bd1\u800c\u975e\u76f4\u8bd1\u3002<\/strong><\/p>\n<p>\u6211\u5df2\u7ecf\u4f7f\u7528ES2015(ES6)\u7684\u8bed\u6cd5\u7f16\u5199JavaScript\u7a0b\u5e8f\u5f88\u4e45\u4e86\uff0c\u5e76\u4e14\u559c\u6b22\u4e0a\u5b83\u63d0\u4f9b\u7684\u65b0\u7279\u6027\u5e26\u6765\u7684\u4f18\u96c5\u548c\u7b80\u6d01\u3002\u6211\u6700\u4e60\u60ef\u7684\u5c31\u662f\u4e0d\u518d\u4f7f\u7528<code>var<\/code>,\u800c\u662f<code>let\/const<\/code>\u3002\u6211\u60f3\u5f53\u7136\u7684\u4ee5\u4e3a<code>let<\/code>\u4ec5\u4ec5\u662f<code>var<\/code>\u7684\u66ff\u4ee3\u8005\uff0c\u800c\u4e8b\u5b9e\u4e0a<code>let<\/code>\u8fd8\u4e3a\u6211\u4eec\u63d0\u4f9b\u4e86\u66f4\u52a0\u7cbe\u7ec6\u7684\u4f5c\u7528\u57df\u3002<\/p>\n<p>\u6211\u5927\u591a\u6570\u65f6\u5019\u4f7f\u7528\u7684\u53d8\u91cf\u90fd\u662f\u7528<code>const<\/code>\u6765\u58f0\u660e\uff0c\u56e0\u4e3a\u5982\u679c\u5c1d\u8bd5\u5bf9\u4f7f\u7528<code>const<\/code>\u58f0\u660e\u7684\u53d8\u91cf\u8fdb\u884c\u4fee\u6539\uff0c\u5c06\u4f1a\u62a5\u9519\u3002\u8fd9\u53ef\u4ee5\u907f\u514d\u4e0d\u5c0f\u5fc3\u5c06\u4e00\u4e2a\u4e0d\u8be5\u4fee\u6539\u7684\u5e38\u91cf\u503c\u4fee\u6539\u3002\u4f46\u662f\uff0c\u6211\u4eec\u8fd8\u662f\u9700\u8981\u53ef\u4ee5\u58f0\u660e\u53ef\u4ee5\u88ab\u4fee\u6539\u7684\u53d8\u91cf\uff0c\u6bd4\u5982\u5728\u5faa\u73af\u91cc\u9762\u7684\u8ba1\u6570\u5668\uff0c\u6211\u4eec\u9700\u8981\u4e0d\u65ad\u5730\u5bf9\u6539\u53d8\u4e86\u52a01\u3002\u53ef\u662f\u4e3a\u4ec0\u4e48\u6211\u4eec\u4f7f\u7528<code>let<\/code>\u800c\u4e0d\u662f<code>var<\/code>\u5462\uff1f<\/p>\n<p>\u6700\u7b80\u5355\u7684\u7b54\u6848\u5c31\u662f<code>let<\/code>\u63d0\u4f9b\u5757\u4f5c\u7528\u57df(block-scoping)\uff0c\u8fd9\u4f1a\u6bd4<code>var<\/code>\u63d0\u4f9b\u7684\u4ee5\u51fd\u6570\u4e3a\u4f5c\u7528\u57df\u6709\u66f4\u52a0\u7cbe\u7ec6\u5316\u7684\u63a7\u5236\u3002\u4e3a\u4e86\u4fbf\u4e8e\u7406\u89e3\uff0c\u6211\u6765\u7528\u4e00\u4e2a\u7ecf\u5178\u7684\u524d\u7aef\u5de5\u7a0b\u5e08\u9762\u8bd5\u7684\u95ee\u9898\u6765\u63cf\u8ff0\u4e24\u8005\u7684\u533a\u522b\u3002<\/p>\n<p>\u95ee\u9898: \u5728\u4e0b\u9762\u7684\u4f8b\u5b50\u4e2d\uff0c\u8bf7\u8bf4\u51fa\u63a7\u5236\u53f0\u7684\u6253\u5370\u7ed3\u679c\u3002<\/p>\n<pre class=\"prettyprint\"><code class=\"language-js hljs  has-numbering\"><span class=\"hljs-keyword\">var<\/span> callbacks = [];\r\n(<span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">()<\/span> {<\/span>\r\n  <span class=\"hljs-keyword\">for<\/span> (<span class=\"hljs-keyword\">var<\/span> i = <span class=\"hljs-number\">0<\/span>; i &lt; <span class=\"hljs-number\">5<\/span>; i++) {\r\n    callbacks.push( <span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">()<\/span> {<\/span> <span class=\"hljs-keyword\">return<\/span> i; } );\r\n  }\r\n})();\r\nconsole.log(callbacks.map( <span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">(cb)<\/span> {<\/span> <span class=\"hljs-keyword\">return<\/span> cb(); } ));<\/code><\/pre>\n<p>\u6211\u4eec\u5c06for\u5faa\u73af\u6267\u884c\u4e94\u6b21\uff0c\u6bcf\u6b21\u5c06\u4e00\u4e2a\u51fd\u6570push\u5230<code>callbacks<\/code>\u6570\u7ec4\u4e2d\u3002\u6700\u540e<code>callbacks<\/code>\u6570\u7ec4\u91cc\u9762\u7684\u6bcf\u4e00\u4e2a\u51fd\u6570\u7684\u6267\u884c\u7ed3\u679c\u6253\u5370\u51fa\u6765\u3002<br \/>\n\u4e00\u4e2a\u65b0\u624b\u5de5\u7a0b\u5e08\u7ecf\u8fc7\u6df1\u601d\u719f\u8651\u53ef\u80fd\u4f1a\u56de\u7b54<code>[0, 1 , 2, 3, 4]<\/code>, \u7136\u800c\u5374\u6389\u5165\u4e86<code>JavaScript<\/code>\u7684\u201dhoisting\u9677\u9631\u201d\u3002<\/p>\n<p>\u53ea\u6709\u5f53\u4f60\u7406\u89e3\u4e86hoisting\uff0c \u624d\u80fd\u7ed9\u51fa\u6b63\u786e\u7684\u7b54\u6848<code>[5, 5, 5, 5, 5]<\/code>\u3002<\/p>\n<pre class=\"prettyprint\"><code class=\"language-js hljs  has-numbering\"><span class=\"hljs-keyword\">var<\/span> callbacks = [];\r\n(<span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">()<\/span> {<\/span>\r\n  <span class=\"hljs-keyword\">var<\/span> i;\r\n  <span class=\"hljs-keyword\">for<\/span> (i = <span class=\"hljs-number\">0<\/span>; i &lt; <span class=\"hljs-number\">5<\/span>; i++) {\r\n    callbacks.push( <span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">()<\/span> {<\/span> <span class=\"hljs-keyword\">return<\/span> i; } );\r\n  }\r\n})();\r\nconsole.log(callbacks.map( <span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">(cb)<\/span> {<\/span> <span class=\"hljs-keyword\">return<\/span> cb(); } ));<\/code><\/pre>\n<p>\u6ce8\u610f\u4e0a\u9762\u7684\u4ee3\u7801\uff0c<code>JavaScript<\/code>\u5c06\u53d8\u91cf\u63d0\u5347\u5230\u51fd\u6570\u5b9a\u4e49\u7684\u9876\u90e8\uff0c\u7ecf\u8fc7\u6574\u4e2a<code>for<\/code>\u5faa\u73af\uff0c<code>callbacks<\/code>\u91cc\u9762\u5b58\u50a8\u76845\u4e2a\u51fd\u6570\u6307\u5411\u7684\u540c\u4e00\u4e2a\u53d8\u91cf<code>i<\/code>\u7684\u503c\u5df2\u7ecf\u662f5\u3002\u6240\u4ee5\u6700\u7ec8\u6253\u5370\u51fa\u6765\u7684\u503c\u90fd\u4e3a5\u3002<\/p>\n<p>\u5728\u4ee5\u524d\u8981\u901a\u8fc7\u5404\u79cd\u5947\u6deb\u6280\u5de7\u6765\u89e3\u51b3\u8fd9\u4e2a\u95ee\u9898\uff0c\u5e76\u6210\u529f\u8fd4\u56de<code>[0, 1, 2, 3, 4]<\/code>, \u73b0\u5728\u6211\u4eec\u6709\u4e86<code>let<\/code>\uff0c\u5c31\u53ef\u4ee5\u5f88\u7b80\u5355\u89e3\u51b3\u95ee\u9898\uff1a<\/p>\n<pre class=\"prettyprint\"><code class=\"language-js hljs  has-numbering\"><span class=\"hljs-keyword\">var<\/span> callbacks = [];\r\n(<span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">()<\/span> {<\/span>\r\n  <span class=\"hljs-keyword\">for<\/span> (<span class=\"hljs-keyword\">let<\/span> i = <span class=\"hljs-number\">0<\/span>; i &lt; <span class=\"hljs-number\">5<\/span>; i++) {\r\n    callbacks.push( <span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">()<\/span> {<\/span> <span class=\"hljs-keyword\">return<\/span> i; } );\r\n  }\r\n})();\r\nconsole.log(callbacks.map( <span class=\"hljs-function\"><span class=\"hljs-keyword\">function<\/span><span class=\"hljs-params\">(cb)<\/span> {<\/span> <span class=\"hljs-keyword\">return<\/span> cb(); } ));<\/code><\/pre>\n<p>\u56e0\u4e3a<code>let<\/code>\u62e5\u6709\u5757\u4f5c\u7528\u57df\uff0c\u6240\u4ee5\u4f7f\u7528<code>let<\/code>\u58f0\u660e\u7684\u53d8\u91cf<code>i<\/code>\u4e0d\u4f1a\u88ab\u63d0\u5347\u5230\u51fd\u6570\u9876\u90e8\uff0c<code>i<\/code>\u7684\u4f5c\u7528\u57df\u5728<code>for<\/code>\u5faa\u73af\uff0c \u5c31\u4f1a\u6bcf\u6b21\u5faa\u73af\u6709\u72ec\u7acb\u7684\u503c\u3002<\/p>\n<p>\u90a3\u6211\u4eec\u662f\u4e0d\u662f\u5e94\u8be5\u4e0d\u8981\u4f7f\u7528<code>var<\/code>\u4e86\u5462\uff1f\u5982\u679c\u4f60\u60f3\u8981\u4e00\u4e2a\u53d8\u91cf\u62e5\u6709\u51fd\u6570\u4f5c\u7528\u57df\uff0c<code>var<\/code>\u8fd8\u662f\u5f88\u6709\u7528\u7684\u3002<\/p>\n<h4 id=\"\u8bfb\u8005\u63d0\u5230\u7684\u4e24\u4e2a\u95ee\u9898\">\u8bfb\u8005\u63d0\u5230\u7684\u4e24\u4e2a\u95ee\u9898\uff1a<\/h4>\n<ul>\n<li><code>const<\/code>\u58f0\u660e\u7684\u53d8\u91cf\u4e0d\u662f\u5b8c\u5168\u4e0d\u53ef\u66f4\u6539\u3002\u6bd4\u5982\uff1a\n<pre class=\"prettyprint\"><code class=\"language-js hljs  has-numbering\"><span class=\"hljs-keyword\">const<\/span> myNotQuiteImmutableObject = {\r\nthisCanBeChanged: <span class=\"hljs-string\">\"not immutable\"<\/span>\r\n};\r\nmyNotQuiteImmutableObject.thisCanBeChanged = <span class=\"hljs-string\">\"see I changed it.\"<\/span>;<\/code><\/pre>\n<p>\u4f46\u662f\uff0c\u4f7f\u7528<code>const<\/code>\u58f0\u660e\u53ef\u4ee5\u963b\u6b62\u4e00\u4e9b\u57fa\u672c\u7684\u66f4\u6539\uff0c\u6bd4\u5982\uff1a<\/p>\n<pre class=\"prettyprint\"><code class=\"language-js hljs  has-numbering\"><span class=\"hljs-keyword\">const<\/span> immutableString = <span class=\"hljs-string\">\"you can't change me\"<\/span>;\r\nimmutableString = <span class=\"hljs-string\">\"D'OH!\"<\/span>; <span class=\"hljs-comment\">\/\/ error<\/span><\/code><\/pre>\n<p>\u5982\u679c\u4f60\u60f3\u8981\u5b8c\u5168\u7684\u4e0d\u53ef\u66f4\u6539\uff0c\u53ef\u4ee5\u4f7f\u7528Facebook\u63d0\u4f9b\u7684Immutable\u5e93\u3002<\/li>\n<li>\u8001\u7248\u672c\u7684\u6d4f\u89c8\u5668\u4e0d\u652f\u6301<code>let<\/code>\u3002\u4e0d\u4ec5\u5982\u6b64\uff0c\u800c\u4e14\u6709\u4e9b\u6700\u65b0\u7684\u6d4f\u89c8\u5668\u4e5f\u8fd8\u6ca1\u6709\u652f\u6301<code>let<\/code>\u3002\u6211\u4eec\u53ef\u4ee5\u4f7f\u7528<code>Babel<\/code>\u6765\u907f\u514d\u8fd9\u4e2a\u95ee\u9898\uff0c<code>Babel<\/code>\u5141\u8bb8\u4f60\u4f7f\u7528\u6240\u6709\u6700\u65b0\u7684JavaScript\u529f\u80fd\uff0c\u7136\u540e\u5c06\u5176\u7ffb\u8bd1\u5230\u751a\u81f3IE8\u90fd\u80fd\u652f\u6301\u7684\u4ee3\u7801\u3002<\/li>\n<\/ul>\n<p class=\"post-copyright\">\u6b22\u8fce\u5206\u4eab\u672c\u6587\uff0c\u8f6c\u8f7d\u8bf7\u4fdd\u7559\u51fa\u5904\uff1a<a href=\"http:\/\/www.qdabc.cn\">\u524d\u7aefABC<\/a> &raquo; <a href=\"http:\/\/www.qdabc.cn\/?p=2234\">ES6\u4e4b&#8221;let&#8221;\u80fd\u66ff\u4ee3&#8221;var&#8221;\u5417?<\/a><\/p>","protected":false},"excerpt":{"rendered":"<p>\u4e3a\u4e86\u4fdd\u8bc1\u53ef\u8bfb\u6027\uff0c\u672c\u6587\u91c7\u7528\u610f\u8bd1\u800c\u975e\u76f4\u8bd1\u3002 \u6211\u5df2\u7ecf\u4f7f\u7528ES2015(ES6)\u7684\u8bed\u6cd5\u7f16\u5199JavaScript\u7a0b\u5e8f\u5f88\u4e45\u4e86\uff0c\u5e76\u4e14\u559c\u6b22\u4e0a\u5b83\u63d0\u4f9b\u7684\u65b0\u7279\u6027\u5e26\u6765\u7684\u4f18\u96c5\u548c\u7b80\u6d01\u3002\u6211\u6700\u4e60\u60ef\u7684\u5c31\u662f\u4e0d\u518d\u4f7f\u7528var,\u800c\u662flet\/c [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[143,155,154],"_links":{"self":[{"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=\/wp\/v2\/posts\/2234"}],"collection":[{"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2234"}],"version-history":[{"count":1,"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=\/wp\/v2\/posts\/2234\/revisions"}],"predecessor-version":[{"id":2235,"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=\/wp\/v2\/posts\/2234\/revisions\/2235"}],"wp:attachment":[{"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2234"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2234"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.qdabc.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2234"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}