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                ThinkChat2.0新版上線,更智能更精彩,支持會話、畫圖、視頻、閱讀、搜索等,送10W Token,即刻開啟你的AI之旅 廣告
                [TOC] ## typeof 區別 `number, string, object, boolean, function, undefined, symbol `這七種類型 ``` typeof 123 // "number" typeof '123' // "string" typeof false // "boolean" typeof undefined typeof window // "object" typeof {} // "object" typeof [] // "object" typeof null // "object" 歷史原因 ``` ## instanceof 判斷是否為實例 instanceof 是用來判斷 A 是否為 B 的實例,**instanceof 檢測的是原** ``` [] instanceof Array; // true {} instanceof Object;// true [] instanceof Object; // true new Date() instanceof Date;// true function Person(){}; new Person() instanceof Person; ``` ### 判斷為聲明的js變量 ``` // 錯誤的寫法 if (v) { // ... } // 報錯: v is not defined // 正確的寫法 if (typeof v === "undefined") { // ... } ```
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