{"id":15034,"date":"2024-10-11T02:24:06","date_gmt":"2024-10-10T18:24:06","guid":{"rendered":"https:\/\/fgchen.com\/wpedu\/?p=15034"},"modified":"2026-03-30T14:29:19","modified_gmt":"2026-03-30T06:29:19","slug":"javascript-%e7%9a%84-var%e3%80%81let-%e5%92%8c-const","status":"publish","type":"post","link":"https:\/\/fgchen.com\/wpedu\/2024\/10\/javascript-%e7%9a%84-var%e3%80%81let-%e5%92%8c-const\/","title":{"rendered":"JavaScript \u7684 var\u3001let \u548c const"},"content":{"rendered":"<p>JavaScript \u7684 var\u3001let \u548c const<\/p>\n<ol>\n<li>var<\/li>\n<\/ol>\n<p>var \u662f\u6700\u65e9\u7684\u8b8a\u6578\u5ba3\u544a\u65b9\u5f0f\u3002\u5b83\u6709\u4ee5\u4e0b\u7279\u9ede\uff1a<\/p>\n<ul>\n<li>\u51fd\u5f0f\u4f5c\u7528\u57df\u6216\u5168\u57df\u4f5c\u7528\u57df<\/li>\n<li>\u53ef\u4ee5\u91cd\u8907\u5ba3\u544a<\/li>\n<li>\u5b58\u5728\u8b8a\u6578\u7f6e\u9802(hoisting)<\/li>\n<\/ul>\n<p>\u793a\u4f8b\uff1a<\/p>\n<pre><code class=\"language-javascript\">console.log(x); \/\/ \u8f38\u51fa\uff1aundefined\uff08\u800c\u975e\u5831\u932f\uff09\nvar x = 5;\n\nfunction example() {\n    var y = 10;\n    if (true) {\n        var y = 20; \/\/ \u540c\u4e00\u500by\n        console.log(y); \/\/ \u8f38\u51fa\uff1a20\n    }\n    console.log(y); \/\/ \u8f38\u51fa\uff1a20\n}\n\nexample();\nconsole.log(x); \/\/ \u8f38\u51fa\uff1a5\nconsole.log(y); \/\/ \u5831\u932f\uff1ay is not defined<\/code><\/pre>\n<p>\u5728\u9019\u500b\u4f8b\u5b50\u4e2d,\u7b2c\u4e00\u884c <code>console.log(x)<\/code> \u4e0d\u6703\u5831\u932f,\u800c\u662f\u8f38\u51fa undefined,\u9019\u5c31\u662f\u8b8a\u6578\u7f6e\u9802\u7684\u6548\u679c\u3002<\/p>\n<ol start=\"2\">\n<li>let<\/li>\n<\/ol>\n<p>let \u662f ES6 \u5f15\u5165\u7684\u8b8a\u6578\u5ba3\u544a\u65b9\u5f0f\u3002\u7279\u9ede\u5982\u4e0b\uff1a<\/p>\n<ul>\n<li>\u5340\u584a\u4f5c\u7528\u57df<\/li>\n<li>\u540c\u4e00\u4f5c\u7528\u57df\u5167\u4e0d\u53ef\u91cd\u8907\u5ba3\u544a<\/li>\n<li>\u4e0d\u5b58\u5728\u8b8a\u6578\u7f6e\u9802<\/li>\n<\/ul>\n<p>\u793a\u4f8b\uff1a<\/p>\n<pre><code class=\"language-javascript\">console.log(a); \/\/ \u5831\u932f\uff1aCannot access &#039;a&#039; before initialization\nlet a = 5;\n\nfunction example() {\n    let b = 10;\n    if (true) {\n        let b = 20; \/\/ \u65b0\u7684b,\u8207\u5916\u5c64\u7684b\u4e0d\u540c\n        console.log(b); \/\/ \u8f38\u51fa\uff1a20\n    }\n    console.log(b); \/\/ \u8f38\u51fa\uff1a10\n}\n\nexample();\nconsole.log(a); \/\/ \u8f38\u51fa\uff1a5\nconsole.log(b); \/\/ \u5831\u932f\uff1ab is not defined<\/code><\/pre>\n<p>\u9019\u88e1,let \u5ba3\u544a\u7684\u8b8a\u6578\u4e0d\u6703\u88ab\u7f6e\u9802,\u56e0\u6b64\u5728\u5ba3\u544a\u524d\u4f7f\u7528\u6703\u5c0e\u81f4\u932f\u8aa4\u3002<\/p>\n<ol start=\"3\">\n<li>const<\/li>\n<\/ol>\n<p>const \u4e5f\u662f ES6 \u5f15\u5165\u7684,\u7528\u65bc\u5ba3\u544a\u5e38\u6578\u3002\u7279\u9ede\u5982\u4e0b\uff1a<\/p>\n<ul>\n<li>\u5340\u584a\u4f5c\u7528\u57df<\/li>\n<li>\u5ba3\u544a\u6642\u5fc5\u9808\u521d\u59cb\u5316<\/li>\n<li>\u4e0d\u80fd\u91cd\u65b0\u8ce6\u503c,\u4f46\u5c0d\u65bc\u53c3\u8003\u578b\u5225,\u5176\u5c6c\u6027\u53ef\u4ee5\u4fee\u6539<\/li>\n<\/ul>\n<p>\u793a\u4f8b\uff1a<\/p>\n<pre><code class=\"language-javascript\">const PI = 3.14159;\nPI = 3; \/\/ \u5831\u932f\uff1aAssignment to a constant variable\n\nconst user = { name: &#039;John&#039;, age: 30 };\nuser.age = 31; \/\/ \u9019\u662f\u5141\u8a31\u7684\nconsole.log(user); \/\/ \u8f38\u51fa\uff1a{ name: &#039;John&#039;, age: 31 }\n\nuser = { name: &#039;Jane&#039; }; \/\/ \u5831\u932f\uff1aAssignment to a constant variable\n\nif (true) {\n    const PI = 3; \/\/ \u9019\u662f\u5141\u8a31\u7684,\u56e0\u70ba\u662f\u65b0\u7684\u4f5c\u7528\u57df\n    console.log(PI); \/\/ \u8f38\u51fa\uff1a3\n}\nconsole.log(PI); \/\/ \u8f38\u51fa\uff1a3.14159<\/code><\/pre>\n<p>const \u8207 let \u985e\u4f3c,\u4e5f\u4e0d\u5b58\u5728\u8b8a\u6578\u7f6e\u9802\u3002<\/p>\n<p>\u7d9c\u5408\u5be6\u4f8b\uff1a<\/p>\n<pre><code class=\"language-javascript\">function scopeExample() {\n    console.log(varVariable); \/\/ \u8f38\u51fa\uff1aundefined\uff08\u8b8a\u6578\u7f6e\u9802\uff09\n    \/\/ console.log(letVariable); \/\/ \u5982\u679c\u53d6\u6d88\u8a3b\u89e3,\u6703\u5831\u932f\n    \/\/ console.log(constVariable); \/\/ \u5982\u679c\u53d6\u6d88\u8a3b\u89e3,\u6703\u5831\u932f\n\n    var varVariable = &#039;\u6211\u662f var&#039;;\n    let letVariable = &#039;\u6211\u662f let&#039;;\n    const constVariable = &#039;\u6211\u662f const&#039;;\n\n    if (true) {\n        var varVariable = &#039;\u6211\u662f\u65b0\u7684 var&#039;;\n        let letVariable = &#039;\u6211\u662f\u65b0\u7684 let&#039;;\n        const constVariable = &#039;\u6211\u662f\u65b0\u7684 const&#039;;\n\n        console.log(varVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u662f\u65b0\u7684 var\n        console.log(letVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u662f\u65b0\u7684 let\n        console.log(constVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u662f\u65b0\u7684 const\n    }\n\n    console.log(varVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u662f\u65b0\u7684 var\n    console.log(letVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u662f let\n    console.log(constVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u662f const\n\n    varVariable = &#039;\u6211\u53ef\u4ee5\u6539\u8b8a&#039;;\n    letVariable = &#039;\u6211\u4e5f\u53ef\u4ee5\u6539\u8b8a&#039;;\n    \/\/ constVariable = &#039;\u6211\u4e0d\u80fd\u6539\u8b8a&#039;; \/\/ \u9019\u6703\u5831\u932f\n\n    console.log(varVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u53ef\u4ee5\u6539\u8b8a\n    console.log(letVariable);  \/\/ \u8f38\u51fa\uff1a\u6211\u4e5f\u53ef\u4ee5\u6539\u8b8a\n}\n\nscopeExample();<\/code><\/pre>\n<p>\u9019\u500b\u7d9c\u5408\u5be6\u4f8b\u5c55\u793a\u4e86 var\u3001let \u548c const \u5728\u4f5c\u7528\u57df\u3001\u53ef\u8b8a\u6027\u548c\u8b8a\u6578\u7f6e\u9802\u65b9\u9762\u7684\u5dee\u7570\u3002var \u7684\u51fd\u5f0f\u4f5c\u7528\u57df\u548c\u8b8a\u6578\u7f6e\u9802\u7279\u6027\u4f7f\u5176\u884c\u70ba\u4e0d\u540c\u65bc let \u548c const\u3002let \u548c const \u5177\u6709\u5340\u584a\u4f5c\u7528\u57df,\u4e0d\u6703\u88ab\u7f6e\u9802,\u4f46 const \u5ba3\u544a\u7684\u8b8a\u6578\u4e0d\u80fd\u88ab\u91cd\u65b0\u8ce6\u503c\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"<p>JavaScript \u7684 var\u3001let \u548c const var var \u662f\u6700\u65e9 &hellip; <\/p>\n","protected":false},"author":1,"featured_media":14707,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_import_markdown_pro_load_document_selector":0,"_import_markdown_pro_submit_text_textarea":"","fifu_image_url":"","fifu_image_alt":"","footnotes":""},"categories":[266],"tags":[],"class_list":["post-15034","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-266"],"_links":{"self":[{"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/posts\/15034","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/comments?post=15034"}],"version-history":[{"count":1,"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/posts\/15034\/revisions"}],"predecessor-version":[{"id":15035,"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/posts\/15034\/revisions\/15035"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/media\/14707"}],"wp:attachment":[{"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/media?parent=15034"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/categories?post=15034"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/fgchen.com\/wpedu\/wp-json\/wp\/v2\/tags?post=15034"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}