{"id":63477,"date":"2023-11-25T11:44:20","date_gmt":"2023-11-25T03:44:20","guid":{"rendered":"https:\/\/docs.pingcode.com\/?p=63477"},"modified":"2023-11-25T11:44:47","modified_gmt":"2023-11-25T03:44:47","slug":"javascript-%e4%b8%ad-var-%e5%92%8c-let-%e7%9a%84%e5%8c%ba%e5%88%ab","status":"publish","type":"post","link":"https:\/\/docs.pingcode.com\/ask\/63477.html","title":{"rendered":"JavaScript \u4e2d var \u548c let \u7684\u533a\u522b"},"content":{"rendered":"\n<blockquote class=\"wp-block-quote\">\n<p>JavaScript\u4e2d\u7684\u53d8\u91cf\u58f0\u660e\u5173\u952e\u5b57\u6709\u591a\u79cd\uff0c\u5176\u4e2d\u5305\u62ec\u4e86var\u548clet\u3002\u867d\u7136\u5b83\u4eec\u90fd\u7528\u4e8e\u58f0\u660e\u53d8\u91cf\uff0c\u4f46\u5b83\u4eec\u4e4b\u95f4\u5b58\u5728\u4e00\u4e9b\u91cd\u8981\u7684\u533a\u522b\u3002\u5305\u62ec\uff1a1.\u4f5c\u7528\u57df\u7684\u533a\u522b\uff1b2.\u53d8\u91cf\u63d0\u5347\u7684\u5dee\u5f02\uff1b3.\u91cd\u590d\u58f0\u660e\u7684\u5904\u7406\u65b9\u5f0f\uff1b4.\u5168\u5c40\u5bf9\u8c61\u5c5e\u6027\uff1b5.\u4e34\u65f6\u6b7b\u533a\uff1b6.\u5faa\u73af\u4e2d\u7684\u884c\u4e3a\u3002\u672c\u6587\u5c06\u6df1\u5165\u63a2\u8ba8var\u548clet\u4e4b\u95f4\u7684\u4e0d\u540c\u4e4b\u5904\uff0c\u4ee5\u5e2e\u52a9\u60a8\u66f4\u597d\u5730\u7406\u89e3\u5b83\u4eec\u7684\u4f7f\u7528\u573a\u666f\u548c\u884c\u4e3a\u3002<\/p>\n<\/blockquote>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>1.<\/strong><strong>\u4f5c\u7528\u57df\u7684\u533a\u522b<\/strong><\/h2>\n\n\n\n<p>var\u58f0\u660e\u7684\u53d8\u91cf\u5c5e\u4e8e\u51fd\u6570\u4f5c\u7528\u57df\uff08function scope\uff09\uff0c\u800clet\u58f0\u660e\u7684\u53d8\u91cf\u5c5e\u4e8e\u5757\u7ea7\u4f5c\u7528\u57df\uff08block scope\uff09\u3002<\/p>\n\n\n\n<p>\u51fd\u6570\u4f5c\u7528\u57df\u610f\u5473\u7740var\u58f0\u660e\u7684\u53d8\u91cf\u5728\u51fd\u6570\u5185\u90e8\u90fd\u662f\u53ef\u89c1\u7684\uff0c\u800c\u5757\u7ea7\u4f5c\u7528\u57df\u610f\u5473\u7740let\u58f0\u660e\u7684\u53d8\u91cf\u53ea\u5728\u5305\u542b\u5b83\u4eec\u7684\u5757\u4e2d\u53ef\u89c1\u3002<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>2.<\/strong><strong>\u53d8\u91cf\u63d0\u5347\u7684\u5dee\u5f02<\/strong><\/h2>\n\n\n\n<p>\u4f7f\u7528var\u58f0\u660e\u7684\u53d8\u91cf\u4f1a\u53d1\u751f\u53d8\u91cf\u63d0\u5347\uff08hoisting\uff09\uff0c\u5373\u5b83\u4eec\u5728\u51fd\u6570\u6216\u5757\u7684\u9876\u90e8\u58f0\u660e\uff0c\u4f46\u521d\u59cb\u5316\u7684\u503c\u4e3aundefined\u3002<\/p>\n\n\n\n<p>\u4f7f\u7528let\u58f0\u660e\u7684\u53d8\u91cf\u4e5f\u4f1a\u53d1\u751f\u53d8\u91cf\u63d0\u5347\uff0c\u4f46\u521d\u59cb\u5316\u7684\u503c\u4e0d\u4f1a\u662fundefined\uff0c\u800c\u662f\u5728\u5757\u5185\u7684\u5b9e\u9645\u58f0\u660e\u4f4d\u7f6e\u4e4b\u524d\u65e0\u6cd5\u8bbf\u95ee\u3002<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>3.\u91cd\u590d\u58f0\u660e\u7684\u5904\u7406\u65b9\u5f0f<\/strong><\/h2>\n\n\n\n<p>\u4f7f\u7528var\u53ef\u4ee5\u591a\u6b21\u58f0\u660e\u540c\u540d\u53d8\u91cf\u800c\u4e0d\u4f1a\u5f15\u53d1\u9519\u8bef\uff0c\u53d8\u91cf\u4f1a\u88ab\u8986\u76d6\u3002<\/p>\n\n\n\n<p>\u4f7f\u7528let\u5728\u540c\u4e00\u4f5c\u7528\u57df\u5185\u591a\u6b21\u58f0\u660e\u540c\u540d\u53d8\u91cf\u4f1a\u5f15\u53d1\u9519\u8bef\u3002<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>4.<\/strong><strong>\u5168\u5c40\u5bf9\u8c61\u5c5e\u6027<\/strong><\/h2>\n\n\n\n<p>\u4f7f\u7528var\u58f0\u660e\u7684\u5168\u5c40\u53d8\u91cf\u4f1a\u6210\u4e3a\u5168\u5c40\u5bf9\u8c61\u7684\u5c5e\u6027\uff0c\u5982window\u5bf9\u8c61\u4e2d\u7684\u5c5e\u6027\u3002<\/p>\n\n\n\n<p>\u4f7f\u7528let\u58f0\u660e\u7684\u5168\u5c40\u53d8\u91cf\u4e0d\u4f1a\u6210\u4e3a\u5168\u5c40\u5bf9\u8c61\u7684\u5c5e\u6027\uff0c\u8fd9\u53ef\u4ee5\u51cf\u5c11\u5168\u5c40\u547d\u540d\u51b2\u7a81\u7684\u53ef\u80fd\u6027\u3002<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>5.<\/strong><strong>\u4e34\u65f6\u6b7b\u533a<\/strong><\/h2>\n\n\n\n<p>\u4f7f\u7528let\u58f0\u660e\u7684\u53d8\u91cf\u5b58\u5728\u4e34\u65f6\u6b7b\u533a\uff08Temporal Dead Zone\uff0cTDZ\uff09\uff0c\u5728\u521d\u59cb\u5316\u4e4b\u524d\u8bbf\u95ee\u5b83\u4eec\u4f1a\u5bfc\u81f4ReferenceError\u3002<\/p>\n\n\n\n<p>\u4f7f\u7528var\u58f0\u660e\u7684\u53d8\u91cf\u6ca1\u6709\u4e34\u65f6\u6b7b\u533a\uff0c\u4f46\u8bbf\u95ee\u672a\u521d\u59cb\u5316\u7684var\u53d8\u91cf\u4f1a\u8fd4\u56deundefined\u3002<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>6.<\/strong><strong>\u5faa\u73af\u4e2d\u7684\u884c\u4e3a<\/strong><\/h2>\n\n\n\n<p>\u5728for\u5faa\u73af\u4e2d\u4f7f\u7528var\u58f0\u660e\u7684\u53d8\u91cf\uff0c\u5176\u4f5c\u7528\u57df\u4e3a\u6574\u4e2a\u5faa\u73af\u4f53\uff0c\u53ef\u80fd\u4f1a\u5bfc\u81f4\u610f\u5916\u7684\u884c\u4e3a\u3002<\/p>\n\n\n\n<p>\u5728for\u5faa\u73af\u4e2d\u4f7f\u7528let\u58f0\u660e\u7684\u53d8\u91cf\uff0c\u5176\u4f5c\u7528\u57df\u4ec5\u9650\u4e8e\u6bcf\u6b21\u8fed\u4ee3\uff0c\u66f4\u7b26\u5408\u9884\u671f\u7684\u884c\u4e3a\u3002<\/p>\n\n\n\n<p><img fetchpriority=\"high\" decoding=\"async\" width=\"484\" height=\"300\" src=\"https:\/\/cdn-docs.pingcode.com\/wp-content\/uploads\/2023\/11\/570402-JavaScript\u4e2dvar\u548clet\u7684\u533a\u522b.png\" class=\"aligncenter\" alt=\"JavaScript \u4e2d var \u548c let \u7684\u533a\u522b\" \/><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>\u5e38\u89c1\u95ee\u7b54<\/strong><\/h2>\n\n\n\n<p><strong>1.var<\/strong><strong>\u548clet\u6709\u4ec0\u4e48\u533a\u522b\uff1f<\/strong><\/p>\n\n\n\n<p>\u4e3b\u8981\u533a\u522b\u5728\u4e8e\u5b83\u4eec\u7684\u4f5c\u7528\u57df\u548c\u53d8\u91cf\u63d0\u5347\u884c\u4e3a\u3002var\u58f0\u660e\u7684\u53d8\u91cf\u5177\u6709\u51fd\u6570\u4f5c\u7528\u57df\uff0c\u800clet\u58f0\u660e\u7684\u53d8\u91cf\u5177\u6709\u5757\u7ea7\u4f5c\u7528\u57df\u3002\u6b64\u5916\uff0cvar\u5b58\u5728\u53d8\u91cf\u63d0\u5347\uff0c\u800clet\u4e0d\u5b58\u5728\u53d8\u91cf\u63d0\u5347\u3002<\/p>\n\n\n\n<p><strong>2.<\/strong><strong>\u4ec0\u4e48\u662f\u53d8\u91cf\u63d0\u5347\uff1f<\/strong><\/p>\n\n\n\n<p>\u53d8\u91cf\u63d0\u5347\u662f\u6307\u5728JavaScript\u4e2d\uff0c\u53d8\u91cf\u548c\u51fd\u6570\u58f0\u660e\u4f1a\u88ab\u63d0\u5347\u5230\u5b83\u4eec\u6240\u5728\u4f5c\u7528\u57df\u7684\u9876\u90e8\u3002\u8fd9\u610f\u5473\u7740\u60a8\u53ef\u4ee5\u5728\u58f0\u660e\u4e4b\u524d\u8bbf\u95ee\u5b83\u4eec\uff0c\u4f46\u503c\u4e3aundefined\u3002\u5bf9\u4e8evar\u58f0\u660e\u7684\u53d8\u91cf\uff0c\u8fd9\u79cd\u884c\u4e3a\u5f88\u660e\u663e\uff0c\u4f46\u5bf9\u4e8elet\u58f0\u660e\u7684\u53d8\u91cf\uff0c\u5b58\u5728\u4e34\u65f6\u6b7b\u533a\uff0c\u8bbf\u95ee\u4f1a\u5bfc\u81f4ReferenceError\u3002<\/p>\n\n\n\n<p><strong>3.<\/strong><strong>\u4e3a\u4ec0\u4e48\u8981\u4f7f\u7528let\u800c\u4e0d\u662fvar\uff1f<\/strong><\/p>\n\n\n\n<p>\u4f7f\u7528let\u6709\u52a9\u4e8e\u907f\u514d\u5728\u5757\u7ea7\u4f5c\u7528\u57df\u5185\u5f15\u8d77\u53d8\u91cf\u6c61\u67d3\u548c\u610f\u5916\u7684\u884c\u4e3a\u3002\u5b83\u66f4\u7b26\u5408\u73b0\u4ee3JavaScript\u7684\u6700\u4f73\u5b9e\u8df5\uff0c\u5e76\u4e14\u901a\u5e38\u66f4\u5bb9\u6613\u7ef4\u62a4\u548c\u8c03\u8bd5\u3002<\/p>\n\n\n\n<p><strong>4.<\/strong><strong>\u5728\u54ea\u4e9b\u60c5\u51b5\u4e0b\u5e94\u8be5\u4f7f\u7528var\uff1f<\/strong><\/p>\n\n\n\n<p>\u867d\u7136let\u662f\u66f4\u73b0\u4ee3\u7684\u53d8\u91cf\u58f0\u660e\u65b9\u5f0f\uff0c\u4f46\u5728\u4e00\u4e9b\u65e7\u7248JavaScript\u73af\u5883\u4e2d\uff0c\u53ef\u80fd\u9700\u8981\u4f7f\u7528var\u3002\u6b64\u5916\uff0c\u5728\u9700\u8981\u5728\u5168\u5c40\u4f5c\u7528\u57df\u4e2d\u58f0\u660e\u53d8\u91cf\u7684\u60c5\u51b5\u4e0b\uff0c\u4e5f\u53ef\u4ee5\u4f7f\u7528var\u3002<\/p>\n\n\n\n<p><strong>5.let<\/strong><strong>\u662f\u5426\u53ef\u4ee5\u5b8c\u5168\u66ff\u4ee3var\uff1f<\/strong><\/p>\n\n\n\n<p>\u662f\u7684\uff0c\u4ece\u6280\u672f\u4e0a\u8bb2\uff0clet\u53ef\u4ee5\u5b8c\u5168\u66ff\u4ee3var\uff0c\u5e76\u4e14\u901a\u5e38\u88ab\u89c6\u4e3a\u66f4\u5b89\u5168\u548c\u53ef\u63a7\u7684\u9009\u62e9\u3002\u7136\u800c\uff0c\u5728\u4e00\u4e9b\u65e7\u7248JavaScript\u4ee3\u7801\u4e2d\uff0c\u53ef\u80fd\u4ecd\u7136\u4f1a\u770b\u5230var\u7684\u4f7f\u7528\u3002\u5728\u7f16\u5199\u65b0\u4ee3\u7801\u65f6\uff0c\u5efa\u8bae\u4f7f\u7528let\u6216const\u4ee5\u83b7\u5f97\u66f4\u597d\u7684\u4f5c\u7528\u57df\u63a7\u5236\u548c\u4ee3\u7801\u53ef\u8bfb\u6027\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"JavaScript\u4e2d\u7684\u53d8\u91cf\u58f0\u660e\u5173\u952e\u5b57\u6709\u591a\u79cd\uff0c\u5176\u4e2d\u5305\u62ec\u4e86var\u548clet\u3002\u867d\u7136\u5b83\u4eec\u90fd\u7528\u4e8e\u58f0\u660e\u53d8\u91cf\uff0c\u4f46\u5b83\u4eec\u4e4b\u95f4\u5b58\u5728 [&hellip;]","protected":false},"author":5,"featured_media":63484,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[37],"tags":[],"acf":[],"_links":{"self":[{"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/posts\/63477"}],"collection":[{"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/comments?post=63477"}],"version-history":[{"count":0,"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/posts\/63477\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/media\/63484"}],"wp:attachment":[{"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/media?parent=63477"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/categories?post=63477"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/docs.pingcode.com\/wp-json\/wp\/v2\/tags?post=63477"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}