{"id":84900,"date":"2023-10-31T15:54:54","date_gmt":"2023-10-31T15:54:54","guid":{"rendered":"https:\/\/abbyhsia.com\/?p=84900"},"modified":"2025-04-15T11:13:53","modified_gmt":"2025-04-15T11:13:53","slug":"notedifference-between-globallocal-variables-javascript","status":"publish","type":"post","link":"https:\/\/abbyhsia.com\/index.php\/2023\/10\/31\/notedifference-between-globallocal-variables-javascript\/","title":{"rendered":"[Note]Difference between Global&Local Variables-JavaScript"},"content":{"rendered":"\n

Cover credit to Gastaloop 102<\/a><\/p>\n\n\n\n

\u66b8\u89e3\u8b8a\u6578\u5168\u57df\u8207\u4f5c\u7528\u57df\u7bc4\u570d\u4e4b\u524d\uff0c\u5148\u4f86\u8a8d\u8b58\u4e00\u4e0b\u8b8a\u6578\u3002<\/p>\n\n\n\n

\u4f55\u8b02\u8b8a\u6578(Variables)<\/h2>\n\n\n\n

\u5ba3\u544a\u8b8a\u6578(declaring variables) \u53ef\u4ee5\u7406\u89e3\u70ba\u662f\u4e00\u500b\u5bb9\u5668\uff0c\u628a\u5c0d\u61c9\u8cc7\u6599\u88dd\u9032\u53bb\uff0c\u4e4b\u5f8c\u53ef\u4ee5\u5f9e\u8b8a\u6578\u5132\u5b58\u7684\u8cc7\u6599\u53d6\u503c\u6216\u62ff\u4f86\u505a\u8a08\u7b97\u3002<\/p>\n\n\n\n

\u800cJavascript\u7684\u5ba3\u544a(declarations)\u6709\u4e09\u7a2e\u65b9\u5f0f\uff0c\u4e0d\u540c\u95dc\u9375\u5b57\u6709\u4e0d\u540c\u4f5c\u7528\u57df\u548c\u884c\u70ba\uff1a <\/p>\n\n\n\n

var(\u53ef\u91cd\u8907\u5ba3\u544a\u8b8a\u6578\uff09 <\/strong><\/p>\n\n\n\n

let(\u5340\u584a\u5340\u57df\u5ba3\u544a\uff0c\u540c\u5340\u584a\u5340\u57df\u7121\u6cd5\u91cd\u8907\u5ba3\u544a) <\/strong><\/p>\n\n\n\n

const(\u4e0d\u53ef\u8b8a\u5e38\u6578\uff09<\/strong><\/p>\n\n\n\n

\u63a5\u4e0b\u4f86\uff0c\u6703\u8a66\u8005\u7528\u6bd4\u55bb\u4f86\u5f62\u5bb9\u9019\u4e09\u500b\u5ba3\u544a\u8b8a\u6578\uff0c\u4ecb\u7d39\u4e4b\u524d\u5148\u7406\u89e3\u4e00\u4e0b\u5ba3\u544a\u8b8a\u6578\u7684\u7a0b\u5f0f\u78bc\u7d50\u69cb:<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

\u5ba3\u544a\u5f8c\u65b9\u63a5\u8005\u8b58\u5225\u5b57(identifiers)<\/a> \uff0c\u518d\u4f86\u8ce6\u4e88\u503c(Value)\uff0c\u503c\u53ef\u4ee5\u662fString, Number, Boolean, Array, Object, Constant, Null, Undefined, Symbol<\/p>\n\n\n\n

JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
var<\/span> <\/span>movieName<\/span>=<\/span>'\u5409\u523b\u6551\u63f4'<\/span>;<\/span>\/\/String<\/span><\/span>\n<\/span>\nlet<\/span> <\/span>judyAge<\/span>=<\/span>30<\/span>;<\/span>\/\/Number<\/span><\/span>\n<\/span>\nvar<\/span> <\/span>isApple<\/span>=<\/span>true<\/span>;<\/span>\/\/Boolean<\/span><\/span>\n<\/span>\nvar<\/span> <\/span>colorName<\/span>=[<\/span>'black'<\/span>,<\/span>'white'<\/span>,<\/span>'gray'<\/span>]; <\/span>\/\/Array<\/span><\/span>\n<\/span>\nvar<\/span> <\/span>person<\/span>={<\/span>name:<\/span>'chuhaha'<\/span>, <\/span>age:<\/span>3<\/span>};<\/span>\/\/Object<\/span><\/span>\n<\/span>\nconst<\/span> <\/span>pi<\/span> =<\/span>3.14159<\/span>;<\/span>\/\/Constant<\/span><\/span>\n<\/span>\nvar<\/span> <\/span>nickName<\/span>=<\/span>null<\/span>;<\/span>\/\/Null<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
Var<\/h3>\n\n\n\n
\u5148\u60f3\u50cf\u6709\u4e00\u500b\u81ea\u52a9\u53d6\u8ca8\u6ac3\uff0c\u6709\u5169\u6ac3\u5b50\u8a2d\u5b9a\u76f8\u540c\u7684\u865f\u78bc\uff0c\u7576\u8f38\u5165\u9019\u7d44\u53d6\u8ca8\u865f\u78bc\uff0c\u6ac3\u5b50\u6703\u7167\u6392\u5e8f\u53ea\u6253\u958b\u6700\u5f8c\u4e00\u7b46\u653e\u9032\u53bb\u7bb1\u5b50\u3002<\/p>\n\n\n\n
\u8209\u4f8bvar\u662f\u7bb1\u5b50\uff0c\u6211\u5148\u62ff\u4e86\u4e00\u500b\u7bb1\u5b50\u88dd\u5165\u96dc\u8a8c\u653e\u9032\u6ac3\u5b50\u8a2d\u5b9a\u2019box\u2019 <\/p>\n\n\n
\n
\"\"<\/figure><\/div>\n\n\n
\u63a5\u8005\u5728\u62ff\u4e00\u500b\u7bb1\u5b50\u88dd\u66f8\u653e\u9032\u53e6\u4e00\u500b\u6ac3\u5b50\u8a2d\u5b9a\u2019box\u2019<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
\u73fe\u5728\uff0c\u6709\u5169\u500b\u2019box\u2019\u7684\u6ac3\u5b50\uff0c\u7576\u53d6\u8ca8\u6642\u8f38\u5165\u201cbox\u201d\u53ea\u6703\u958b\u555f\u6700\u5f8c\u653e\u66f8\u7684\u6ac3\u5b50<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
JS\u7a0b\u5f0f\u78bc\u5982\u4e0b\uff1a<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
var<\/span> <\/span>box<\/span>=<\/span>'magazine'<\/span>;<\/span><\/span>\nvar<\/span> <\/span>box<\/span>=<\/span>'book'<\/span>;<\/span><\/span>\n<\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>box<\/span>);<\/span><\/span>\n\/\/output: book<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
let<\/h2>\n\n\n\n
\u5ef6\u7e8c\u81ea\u52a9\u53d6\u8ca8\u6ac3\u4f8b\u5b50\uff0c\u5148\u5c07\u5831\u7d19\u88dd\u7bb1\u5f8c\u653e\u5165\u6ac3\u5b50\u8a2d\u5b9a\u2019box\u2019\uff0c\u723e\u5f8c\u6709\u5176\u5b83\u6ac3\u5b50\u5b58\u653elet\u8a2d\u5b9a\u2018box\u2019\uff0c\u5728\u53d6\u8ca8\u8f38\u5165\u2019box\u2019\u662f\u7121\u6cd5\u958b\u555f\u4efb\u4f55\u4e00\u500b\u6ac3\u5b50\u3002<\/p>\n\n\n\n
\u4e0b\u65b9\u7a0b\u5f0f\u78bc\u7528\u300c\u51fd\u6578\u300d\u505a\u793a\u7bc4<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
function<\/span> <\/span>aArea<\/span>(){<\/span><\/span>\nlet<\/span> <\/span>box<\/span>=<\/span>'newspapres'<\/span>;<\/span><\/span>\nlet<\/span> <\/span>box<\/span>=<\/span>'novels'<\/span>;<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>'\u5c0f\u5409\u53d6\u8ca8\u62ff\u4e86'<\/span>+<\/span>box<\/span>);<\/span><\/span>\n}<\/span><\/span>\naArea<\/span>();<\/span><\/span>\n\/\/ Uncaught SyntaxError: Identifier 'box' has already been declared<\/span><\/span>\n<\/span><\/code><\/pre><\/div>\n\n\n\n
\u4e0d\u904e\uff0cJS\u6709\u5176\u5b83\u91cd\u65b0\u8ce6\u503c\u65b9\u5f0f\uff1a<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
function<\/span> <\/span>aArea<\/span>(){<\/span><\/span>\nlet<\/span> <\/span>box<\/span>=<\/span>'newspapres'<\/span>;<\/span><\/span>\nbox<\/span>=<\/span>'novel'<\/span>;<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>'\u5c0f\u5409\u53d6\u8ca8\u62ff\u4e86'<\/span>+<\/span>box<\/span>);<\/span><\/span>\n}<\/span><\/span>\naArea<\/span>();<\/span><\/span>\n\/\/output: novel<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
const<\/h2>\n\n\n\n
\u8209\u4f8bconst\u662f\u7bb1\u5b50\uff0c\u7e6a\u672c\u88dd\u7bb1\u653e\u5165\u6ac3\u5b50\u8a2d\u5b9a\u2018box\u2019\uff0c\u723e\u5f8c\u51fa\u73fe\u5176\u5b83\u8a2d\u5b9a\u2019box\u2019\u6ac3\u5b50\uff0c\u8f38\u5165\u2018box\u2019\u7686\u7121\u6cd5\u958b\u555f\uff0cconst\u6982\u5ff5\u540clet\u7b2c\u4e00\u9ede\u8aaa\u660e\u6982\u5ff5\uff0c\u4f46\u8207let\u4e0d\u540c\u4e4b\u8655\uff0cconst\u4e0d\u53ef\u7528\u5176\u5b83\u65b9\u5f0f\u91cd\u65b0\u8ce6\u503c\u3002<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
const<\/span> <\/span>box<\/span>=<\/span>'pictureBook'<\/span>;<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>box<\/span>);<\/span><\/span>\n\/\/output: Picturebook<\/span><\/span>\n<\/span>\n<\/span>\nconst<\/span> <\/span>box<\/span>=<\/span>'pictureBook'<\/span>;<\/span><\/span>\nconst<\/span> <\/span>box<\/span>=<\/span>'HistoricalFiction'<\/span>;<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>box<\/span>);<\/span><\/span>\n\/\/Uncaught SyntaxError: Identifier 'box' has already been declared<\/span><\/span>\n<\/span>\n<\/span>\nconst<\/span> <\/span>box<\/span>=<\/span>'pictureBook'<\/span>;<\/span><\/span>\nbox<\/span>=<\/span>'historicalFiction'<\/span>;<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>box<\/span>);<\/span><\/span>\n\/\/Uncaught TypeError: Assignment to constant variable.<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
\u8b8a\u6578\u53d6\u503c(Declaration and initialization)<\/h2>\n\n\n\n
\u9019\u88e1\u5148\u6dfa\u8ac7\u8b8a\u6578\u63d0\u5347(Variable Hoisting)\uff0cJS\u5728\u4ee3\u78bc\u57f7\u884cHoisting\u4e4b\u524d\u6703\u5c07\u8b8a\u6578\u79fb\u5230\u4f5c\u7528\u57df\u4e2d\uff0c\u8b80\u53d6\u7a0b\u5f0f\u78bc\u9047\u5230\u8b8a\u6578\u6642\u8b58\u5225\u5b57\u6703\u63d0\u5347\u81f3\u4f5c\u7528\u57df\u524d\uff0c\u4f46\u8ce6\u503c\u4e0d\u6703\u63d0\u5347\uff0c\u63db\u8a00\u4e4b\uff0c\u8b8a\u6578\u4e2d\u7684\u503c\u5fc5\u9808\u5beb\u5728JS\u4f5c\u7528\u57df\u7bc4\u570d\uff0c\u624d\u80fd\u6709\u6548\u8b80\u53d6\u3002<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
console<\/span>.<\/span>log<\/span>(<\/span>chiHuahua<\/span>);<\/span><\/span>\nvar<\/span> <\/span>chiHuahua<\/span>=<\/span>'\u5409\u5a03\u5a03'<\/span>;<\/span><\/span>\n\/\/undefined<\/span><\/span>\n<\/span>\n<\/span>\nvar<\/span> <\/span>dog<\/span>=<\/span>'\u5409\u5a03\u5a03'<\/span><\/span>\nvar<\/span> <\/span>combine<\/span>=<\/span>dog<\/span>+<\/span>universe<\/span>;<\/span><\/span>\nvar<\/span> <\/span>universe<\/span>=<\/span>'\u88ab\u9010\u51fa\u5b87\u5b99'<\/span>;<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>combine<\/span>);<\/span><\/span>\n\/\/\u5409\u5a03\u5a03undefined<\/span><\/span>\n<\/span>\n<\/span>\nvar<\/span> <\/span>dog<\/span>=<\/span>'\u5409\u5a03\u5a03'<\/span><\/span>\nvar<\/span> <\/span>universe<\/span>=<\/span>'\u88ab\u9010\u51fa\u5b87\u5b99'<\/span>;<\/span><\/span>\nvar<\/span> <\/span>combine<\/span>=<\/span>dog<\/span>+<\/span>universe<\/span>;<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>combine<\/span>);<\/span><\/span>\n\/\/output:\u5409\u5a03\u5a03\u88ab\u9010\u51fa\u5b87\u5b99<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
(\u5b57\u4e32\u793a\u7bc4\u7528\uff0c\u7b46\u8005\u652f\u6301\u751f\u547d\u5e73\u6b0a\uff0c\u52d5\u7269\u53cb\u5584\uff09<\/mark>\ud83d\udc36<\/p>\n\n\n\n
<\/p>\n\n\n\n
\u8b8a\u6578\u7bc4\u570d(Variable Scope)<\/h2>\n\n\n\n
var, let, const \u8b8a\u6578\u8207\u5e38\u6578\u5728\u4e0d\u540c\u4f5c\u7528\u57df\u7684\u4f4d\u7f6e\uff0c\u6703\u5f71\u97ffJS\u8b80\u53d6\u7d50\u679c\uff0c<\/p>\n\n\n\n
\u9019\u88e1\u5206\u70ba\u5169\u7a2e\u5168\u57df\u8b8a\u6578(Global Variables)\u3001\u5340\u57df\u8b8a\u6578(Local Variables)<\/p>\n\n\n\n
\u5168\u57df\u8b8a\u6578(Global Variables)<\/h3>\n\n\n\n
\u8b8a\u6578\u5beb\u5728\u5168\u57df\u5c6c\u6027\u53ef\u4ee5\u8b93\u6574\u500b\u7a0b\u5f0f\u8b80\u53d6\uff0c\u4f46\u8981\u7559\u610f\u7a0b\u5f0f\u78bc\u6578\u91cf\u4e00\u591a\u60c5\u6cc1\u4e0b\u9020\u6210\u547d\u540d\u6df7\u4e82\u3002<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
var<\/span> <\/span>chiHuahua<\/span> =<\/span>'\u5409\u5a03\u5a03'<\/span>;<\/span><\/span>\nvar<\/span> <\/span>aka<\/span> =<\/span>'\u72d7\u754c\u8ff7\u4f60\u6280\u5b89'<\/span>;<\/span><\/span>\n<\/span>\nfunction<\/span> <\/span>combine<\/span>(){<\/span><\/span>\n\t<\/span>console<\/span>.<\/span>log<\/span>(<\/span>'\u6211\u662f\u4e00\u96bb'<\/span>+<\/span>chiHuahua<\/span>+<\/span>'\u53c8\u540d'<\/span>+<\/span>aka<\/span>);<\/span><\/span>\n}<\/span><\/span>\ncombine<\/span>();<\/span><\/span>\n\/\/Output:\u6211\u662f\u4e00\u96bb\u5409\u5a03\u5a03\u53c8\u540d\u72d7\u754c\u8ff7\u4f60\u6280\u5b89<\/span><\/span>\n<\/span>\n<\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>'\u9019\u6a23\u4e5f\u53ef\u4ee5\u8b80\u5230'<\/span>+<\/span>chiHuahua<\/span>+<\/span>aka<\/span>);<\/span><\/span>\n\/\/Output:\u9019\u6a23\u4e5f\u53ef\u4ee5\u8b80\u5230\u5409\u5a03\u5a03\u53c8\u540d\u72d7\u754c\u8ff7\u4f60\u6280\u5b89<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
\u6b64\u5916\uff0c\u5168\u57df\u8b8a\u6578\u7684\u6709\u500b\u60c5\u6cc1\uff0c\u7576\u51fd\u5f0f\u5167\u5beb\u6578\u503c\uff0c\u6b64\u65b9\u5f0f\u540c\u7b49\u5ba3\u544a\u5168\u57df\u8b8a\u6578\uff0c\u5373\u4f7f\u5728\u51fd\u6578\u4e4b\u5916\u4e5f\u53ef\u4ee5\u53d6\u503c\uff0c\u76e1\u91cf\u4f9d\u64da\u60c5\u5883\u8003\u616e\u4f7f\u7528var, let, const\u70ba\u4f73\u3002<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
function<\/span> <\/span>dog<\/span>(){<\/span><\/span>\n\t<\/span>name<\/span>=<\/span>'pug'<\/span>;<\/span><\/span>\n}<\/span><\/span>\ndog<\/span>();<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>name<\/span>);<\/span><\/span>\n\/\/output\uff1apug<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
\u5340\u57df\u8b8a\u6578(Local Variables)<\/h2>\n\n\n\n
\u5340\u57df\u8b8a\u6578\u662f\u6307\u5728\u7279\u5b9a\u5340\u584a\u88ab\u8072\u660e\uff0c\u8b8a\u6578\u53ef\u4ee5\u5beb\u5728\u51fd\u6578\u3001if\u3001\u8ff4\u5708\u3002<\/p>\n\n\n\n
\u4ee5\u4e0b\u8209\u4f8b\uff0c\u51fd\u6578\u5167\u7684var, let, const\u8b8a\u6578\uff0c\u53ea\u6703\u5728\u51fd\u6578\u4e2d\u6709\u4f5c\u7528\uff0c\u5728\u547c\u53eb\u51fd\u6578\u6642\u53ef\u4ee5\u6210\u529f\u53d6\u503c\uff0c\u800c\u8d85\u51fa\u51fd\u6578\u4e4b\u5916\u8a2a\u554f\u88e1\u9762\u8b8a\u6578\u7a0b\u5f0f\u78bc\u6703\u5f97\u5230\u672a\u5b9a\u7fa9\u7684\u7d50\u679c\u3002
\u88dc\u5145\u8aaa\u660e\uff1alet, const\u662f\u5728ECMAScript 6\uff08ES6\uff09\u7684\u7248\u672c\u5f15\u5165\u5340\u57df\u5ba3\u544a\u8072\u660e\uff0cES5\u4e4b\u524d\u7248\u672cvar\u5177\u6709\u5168\u57df\u7279\u6027;\u5373\u4f7f\u5728\u4f5c\u7528\u57df\u7121\u6cd5\u986f\u793a\u7d50\u679c\uff0c\u900f\u904eChrome Inspect\u67e5\u8a62\u9084\u662f\u6709\u5ba3\u544a\u6210\u529f\uff0cES6\u5efa\u8b70\u907f\u514d\u6c61\u67d3\u5168\u57df\u60c5\u6cc1\u4e0b\uff0c{}\u5167\u8acb\u4f7f\u7528let, const\u53d6\u4ee3var\u3002<\/p>\n\n\n\n
JavaScript<\/span><\/span><\/path><\/path><\/svg><\/span>
function<\/span> <\/span>chiHuahua<\/span>() {<\/span><\/span>\n  <\/span>let<\/span> <\/span>dogName<\/span> = <\/span>'\u5409\u5a03\u5a03'<\/span>;<\/span><\/span>\n\t<\/span>let<\/span> <\/span>character<\/span>=<\/span>'\u82e5\u5409\u82e5\u68a8'<\/span><\/span>\n  <\/span>console<\/span>.<\/span>log<\/span>(<\/span>dogName<\/span>+<\/span>character<\/span>);<\/span><\/span>\n}<\/span><\/span>\nchiHuahua<\/span>();<\/span><\/span>\n\/\/output\uff1a\u5409\u5a03\u5a03\u82e5\u5409\u82e5\u68a8<\/span><\/span>\n<\/span>\n<\/span>\nfunction<\/span> <\/span>chiwawa<\/span>() {<\/span><\/span>\n  <\/span>let<\/span> <\/span>dogName<\/span> = <\/span>'\u6b50\u7df4\u5409'<\/span>;<\/span><\/span>\n}<\/span><\/span>\nchiwawa<\/span>();<\/span><\/span>\nconsole<\/span>.<\/span>log<\/span>(<\/span>'\u6709\u96bb\u5409\u5a03\u5a03\u53eb'<\/span> + <\/span>dogName<\/span>);<\/span><\/span>\n\/\/Uncaught ReferenceError: dogName is not defined<\/span><\/span><\/code><\/pre><\/div>\n\n\n\n
\u5076\u723e\u6703\u756b\u5716\u4f86\u7406\u89e3\u7a0b\u5f0f\u78bc\uff0c\u82e5\u5167\u6587\u5716\u50cf\u5c0d\u4f60\u7406\u89e3\u6709\u5e6b\u52a9\uff0c\u6b61\u8fce\u81ea\u884c\u53d6\u7528\u3002<\/p>\n\n\n\n
\u6b64\u70ba\u7d00\u9304\u5b78\u7fd2\u904e\u7a0b\uff0c\u5982\u5167\u5bb9\u6709\u8aa4\uff0c\u6b61\u8fce\u7559\u8a00\u6307\u6559\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"
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