typeof 運算符把類型信息當作字符串返回。typeof 返回值有六種可能： "number," "string," "boolean," "object," "function," 和 "undefined."我們可以使用typeof來獲取一個變量是否存在，如if(typeof a!="undefined"){}，而不要去使用if(a)因為如果a不存在（未聲明）則會出錯，對於Array,Null等特殊對象使用typeof一律返回object，這正是typeof的局限性。
typeof 語法中的圓括號是可選項。
if(document.mylist.length != “undefined” ) {} 這個用法有誤.
正確的是 if( typeof(document.mylist.length) != “undefined” ) {}
或 if( !isNaN(document.mylist.length) ) {}
typeof的運算數未定義,返回的就是 “undefined”.
運算數為數字 typeof(x) = “number”
字符串 typeof(x) = “string”
布爾值 typeof(x) = “boolean”
對象,數組和null typeof(x) = “object”
函數 typeof(x) = “function”
typeof 運算符返回一個用來表示表達式的數據類型的字符串。
可能的字符串有：”number”、”string”、”boolean”、”object”、”function” 和 “undefined”。
如：
| 代碼如下 | 復制代碼 |
| alert(typeof (123));//typeof(123)返回”number” alert(typeof (“123″));//typeof(“123″)返回”string” |
|
對於Array,Null等特殊對象使用typeof一律返回object，這正是typeof的局限性。
如果我們希望獲取一個對象是否是數組，或判斷某個變量是否是某個對象的實例則要選擇使用instanceof。instanceof用於判斷一個變量是否某個對象的實例，如var a=new Array();alert(a instanceof Array);會返回true，同時alert(a instanceof Object)也會返回true;這是因為Array是object的子類。再如：function test(){};var a=new test();alert(a instanceof test)會返回true。
友情提示
a instanceof Object 得到true並不是因為 Array是Object的子對象，而是因為 Array的prototype屬性構造於Object，Array的父級是Function
Examples
// Numbers typeof 37 === 'number'; typeof 3.14 === 'number'; typeof(42) === 'number'; typeof Math.LN2 === 'number'; typeof Infinity === 'number'; typeof NaN === 'number'; // Despite being "Not-A-Number" typeof Number(1) === 'number'; // but never use this form! // Strings typeof "" === 'string'; typeof "bla" === 'string'; typeof (typeof 1) === 'string'; // typeof always returns a string typeof String("abc") === 'string'; // but never use this form! // Booleans typeof true === 'boolean'; typeof false === 'boolean'; typeof Boolean(true) === 'boolean'; // but never use this form! // Symbols typeof Symbol() === 'symbol' typeof Symbol('foo') === 'symbol' typeof Symbol.iterator === 'symbol' // Undefined typeof undefined === 'undefined'; typeof declaredButUndefinedVariable === 'undefined'; typeof undeclaredVariable === 'undefined'; // Objects typeof {a:1} === 'object'; // use Array.isArray or Object.prototype.toString.call // to differentiate regular objects from arrays typeof [1, 2, 4] === 'object'; typeof new Date() === 'object'; // The following is confusing. Don't use! typeof new Boolean(true) === 'object'; typeof new Number(1) === 'object'; typeof new String("abc") === 'object'; // Functions typeof function(){} === 'function'; typeof class C {} === 'function'; typeof Math.sin === 'function';null
// This stands since the beginning of JavaScript typeof null === 'object';In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the bogus typeof return value. (reference)
A fix was proposed for ECMAScript (via an opt-in), but was rejected. It would have resulted intypeof null === 'null'.
Regular expressions
Callable regular expressions were a non-standard addition in some browsers.
typeof /s/ === 'function'; // Chrome 1-12 Non-conform to ECMAScript 5.1 typeof /s/ === 'object'; // Firefox 5+ Conform to ECMAScript 5.1Exceptions
All current browsers expose a non-standard host object document.all with type Undefined.
typeof document.all === 'undefined';Although the specification allows custom type tags for non-standard exotic objects, it requires those type tags to be different from the predefined ones. The case of document.all having type tag'undefined' must be classified as an exceptional violation of the rules.