{"id":1414,"date":"2024-06-02T23:01:05","date_gmt":"2024-06-02T23:01:05","guid":{"rendered":""},"modified":"2024-06-05T06:12:46","modified_gmt":"2024-06-04T22:12:46","slug":"typescript-%e4%b9%8b-narrowing","status":"publish","type":"post","link":"https:\/\/www.lccc.top\/?p=1414","title":{"rendered":"TypeScript \u4e4b Narrowing"},"content":{"rendered":"\n

\u524d\u8a00<\/h2>\n

TypeScript \u7684\u5b98\u65b9\u6587\u6863\u65e9\u5df2\u66f4\u65b0\uff0c\u4f46\u6211\u80fd\u627e\u5230\u7684\u4e2d\u6587\u6587\u6863\u90fd\u8fd8\u505c\u7559\u5728\u6bd4\u8f83\u8001\u7684\u7248\u672c\u3002\u6240\u4ee5\u5bf9\u5176\u4e2d\u65b0\u589e\u53ca\u4fee\u6539\u8f83\u591a\u7684\u7ae0\u8282\u8fdb\u884c\u4e86\u4e2a\u4eba\u7684\u7ffb\u8bd1\u6574\u7406\u3002<\/p>\n

\u672c\u7bc7\u6574\u7406\u81ea https:\/\/www.typescriptlang.org\/docs\/handbook\/2\/narrowing.html<\/a><\/p>\n

\u672c\u6587\u5e76\u4e0d\u5b8c\u5168\u9075\u5faa\u539f\u6587\u7ffb\u8bd1\uff0c\u5bf9\u90e8\u5206\u5185\u5bb9\u81ea\u5df1\u4e5f\u505a\u4e86\u89e3\u91ca\u8865\u5145\u3002<\/p>\n

Narrowing<\/h2>\n

\u8bd5\u60f3\u6211\u4eec\u6709\u8fd9\u6837\u4e00\u4e2a\u51fd\u6570\uff0c\u51fd\u6570\u540d\u4e3a padLeft\uff1a<\/p>\n

<\/p>\n
function<\/span> padLeft<\/span>(<\/span>padding<\/span>: number<\/span> |<\/span> string<\/span>,<\/span> input<\/span>: string<\/span>)<\/span>: string<\/span> {<\/span>\n  throw<\/span> new<\/span> Error<\/span>(<\/span>\"Not implemented yet!\"<\/span>)<\/span>;<\/span>\n}<\/span><\/pre>\n<\/div>\n
\u8be5\u51fd\u6570\u5b9e\u73b0\u7684\u529f\u80fd\u662f\uff1a<\/p>\n
\u5982\u679c\u53c2\u6570 padding<\/code> \u662f\u4e00\u4e2a\u6570\u5b57\uff0c\u6211\u4eec\u5c31\u5728 input<\/code> \u524d\u9762\u6dfb\u52a0\u540c\u7b49\u6570\u91cf\u7684\u7a7a\u683c\uff0c\u800c\u5982\u679c padding<\/code> \u662f\u4e00\u4e2a\u5b57\u7b26\u4e32\uff0c\u6211\u4eec\u5c31\u76f4\u63a5\u6dfb\u52a0\u5230 input<\/code> \u524d\u9762\u3002<\/p>\n
\u8ba9\u6211\u4eec\u5b9e\u73b0\u4e00\u4e0b\u8fd9\u4e2a\u903b\u8f91\uff1a<\/p>\n
<\/p>\n
function<\/span> padLeft<\/span>(<\/span>padding<\/span>: number<\/span> |<\/span> string<\/span>,<\/span> input<\/span>: string<\/span>)<\/span> {<\/span>\n  return<\/span> new<\/span> Array<\/span>(<\/span>padding<\/span> +<\/span> 1<\/span>)<\/span>.<\/span>join<\/span>(<\/span>\" \"<\/span>)<\/span> +<\/span> input<\/span>;<\/span>\n\t\/\/ Operator '+' cannot be applied to types 'string | number' and 'number'.<\/span>\n}<\/span><\/pre>\n<\/div>\n
\u5982\u679c\u8fd9\u6837\u5199\u7684\u8bdd\uff0c\u7f16\u8f91\u5668\u91cc padding + 1<\/code> \u8fd9\u4e2a\u5730\u65b9\u5c31\u4f1a\u6807\u7ea2\uff0c\u663e\u793a\u4e00\u4e2a\u9519\u8bef\u3002
\n\u200b
\n\u8fd9\u662f TypeScript \u5728\u8b66\u544a\u6211\u4eec\uff0c\u5982\u679c\u628a\u4e00\u4e2a number<\/code> \u7c7b\u578b (\u5373\u4f8b\u5b50\u91cc\u7684\u6570\u5b57 1 )\u548c\u4e00\u4e2a number | string<\/code> \u7c7b\u578b\u76f8\u52a0\uff0c\u4e5f\u8bb8\u5e76\u4e0d\u4f1a\u8fbe\u5230\u6211\u4eec\u60f3\u8981\u7684\u7ed3\u679c\u3002\u6362\u53e5\u8bdd\u8bf4\uff0c\u6211\u4eec\u5e94\u8be5\u5148\u68c0\u67e5\u4e0b padding<\/code> \u662f\u5426\u662f\u4e00\u4e2a number<\/code>\uff0c\u6216\u8005\u5904\u7406\u4e0b\u5f53 padding<\/code> \u662f string<\/code> \u7684\u60c5\u51b5\uff0c\u90a3\u6211\u4eec\u53ef\u4ee5\u8fd9\u6837\u505a\uff1a<\/p>\n
<\/p>\n
function<\/span> padLeft<\/span>(<\/span>padding<\/span>: number<\/span> |<\/span> string<\/span>,<\/span> input<\/span>: string<\/span>)<\/span> {<\/span>\n  if<\/span> (<\/span>typeof<\/span> padding<\/span> ===<\/span> \"number\"<\/span>)<\/span> {<\/span>\n    return<\/span> new<\/span> Array<\/span>(<\/span>padding<\/span> +<\/span> 1<\/span>)<\/span>.<\/span>join<\/span>(<\/span>\" \"<\/span>)<\/span> +<\/span> input<\/span>;<\/span>\n  }<\/span>\n  return<\/span> padding<\/span> +<\/span> input<\/span>;<\/span>\n}<\/span><\/pre>\n<\/div>\n
\u8fd9\u4e2a\u4ee3\u7801\u770b\u4e0a\u53bb\u4e5f\u8bb8\u6ca1\u6709\u4ec0\u4e48\u6709\u610f\u601d\u7684\u5730\u65b9\uff0c\u4f46\u5b9e\u9645\u4e0a\uff0cTypeScript \u5728\u80cc\u540e\u505a\u4e86\u5f88\u591a\u4e1c\u897f\u3002<\/p>\n
TypeScript \u8981\u5b66\u7740\u5206\u6790\u8fd9\u4e9b\u4f7f\u7528\u4e86\u9759\u6001\u7c7b\u578b\u7684\u503c\u5728\u8fd0\u884c\u65f6\u7684\u5177\u4f53\u7c7b\u578b\u3002\u76ee\u524d TypeScript \u5df2\u7ecf\u5b9e\u73b0\u4e86\u6bd4\u5982 if\/else<\/code> \u3001\u4e09\u5143\u8fd0\u7b97\u7b26\u3001\u5faa\u73af\u3001\u771f\u503c\u68c0\u67e5\u7b49\u60c5\u51b5\u4e0b\u7684\u7c7b\u578b\u5206\u6790\u3002<\/p>\n
\u5728\u6211\u4eec\u7684 if<\/code> \u8bed\u53e5\u4e2d\uff0cTypeScript \u4f1a\u8ba4\u4e3a typeof padding === number<\/code> \u662f\u4e00\u79cd\u7279\u6b8a\u5f62\u5f0f\u7684\u4ee3\u7801\uff0c\u6211\u4eec\u79f0\u4e4b\u4e3a\u7c7b\u578b\u4fdd\u62a4 (type guard)<\/strong>\uff0cTypeScript \u4f1a\u6cbf\u7740\u6267\u884c\u65f6\u53ef\u80fd\u7684\u8def\u5f84\uff0c\u5206\u6790\u503c\u5728\u7ed9\u5b9a\u7684\u4f4d\u7f6e\u4e0a\u6700\u5177\u4f53\u7684\u7c7b\u578b\u3002<\/p>\n
TypeScript \u7684\u7c7b\u578b\u68c0\u67e5\u5668\u4f1a\u8003\u8651\u5230\u8fd9\u4e9b\u7c7b\u578b\u4fdd\u62a4\u548c\u8d4b\u503c\u8bed\u53e5\uff0c\u800c\u8fd9\u4e2a\u5c06\u7c7b\u578b\u63a8\u5bfc\u4e3a\u66f4\u7cbe\u786e\u7c7b\u578b\u7684\u8fc7\u7a0b\uff0c\u6211\u4eec\u79f0\u4e4b\u4e3a\u6536\u7a84 (narrowing)<\/strong>\u3002 \u5728\u7f16\u8f91\u5668\u4e2d\uff0c\u6211\u4eec\u53ef\u4ee5\u89c2\u5bdf\u5230\u7c7b\u578b\u7684\u6539\u53d8\uff1a<\/p>\n
\"image.png\"<\/a><\/p>\n
\u4ece\u4e0a\u56fe\u4e2d\u53ef\u4ee5\u770b\u5230\u5728 if<\/code> \u8bed\u53e5\u4e2d\uff0c\u548c\u5269\u4f59\u7684 return<\/code> \u8bed\u53e5\u4e2d\uff0cpadding<\/code> \u7684\u7c7b\u578b\u90fd\u63a8\u5bfc\u4e3a\u66f4\u7cbe\u786e\u7684\u7c7b\u578b\u3002<\/p>\n
\u63a5\u4e0b\u6765\uff0c\u6211\u4eec\u5c31\u4ecb\u7ecd narrowing<\/code> \u6240\u6d89\u53ca\u7684\u5404\u79cd\u5185\u5bb9\u3002<\/p>\n
typeof \u7c7b\u578b\u4fdd\u62a4\uff08type guards\uff09<\/h2>\n
JavaScript \u672c\u8eab\u5c31\u63d0\u4f9b\u4e86 typeof<\/code> \u64cd\u4f5c\u7b26\uff0c\u53ef\u4ee5\u8fd4\u56de\u8fd0\u884c\u65f6\u4e00\u4e2a\u503c\u7684\u57fa\u672c\u7c7b\u578b\u4fe1\u606f\uff0c\u4f1a\u8fd4\u56de\u5982\u4e0b\u8fd9\u4e9b\u7279\u5b9a\u7684\u5b57\u7b26\u4e32\uff1a<\/p>\n