typeof

typeof操作符返回一个字符串，指示未经计算的操作数的类型。

语法

该typeof操作是紧跟其操作：

typeof operand

参数

operand 是一个表达式，表示对象或原始值，其类型将被返回。

描述

下表总结了typeof可能的返回值。有关类型和原始值的更多信息，可查看 JavaScript数据结构页面。

Type	Result
Undefined	"undefined"
Null	"object" (see below)
Boolean	"boolean"
Number	"number"
String	"string"
Symbol (new in ECMAScript 2015)	"symbol"
Host object (provided by the JS environment)	Implementation-dependent
Function object (implements [Call] in ECMA-262 terms)	"function"
Any other object	"object"

示例

// 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 '1' === 'string'; // note that a number within a string is still typeof 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';

在 JavaScript 最初的实现中，JavaScript 中的值是由一个表示类型的标签和实际数据值表示的。对象的类型标签是 0。由于 null 代表的是空指针（大多数平台下值为 0x00），因此，null的类型标签也成为了 0，typeof null就错误的返回了"object"。（reference）

ECMAScript提出了一个修复（通过opt-in），但被拒绝。这将导致typeof null === 'object'。

使用 new 操作符

// All constructor function while instaitated with 'new' keyword will always be typeof 'object'
var str = new String('String'
var num = new Number(100

typeof str; // It will return 'object'
typeof num; // It will return 'object'

//But there is a exception in case of Function constructor of Javascript

var func = new Function(

typeof func; // It will return 'function'

正则表达式

对正则表达式字面量的类型判断在某些浏览器中不符合标准：

typeof /s/ === 'function'; // Chrome 1-12 Non-conform to ECMAScript 5.1
typeof /s/ === 'object';   // Firefox 5+  Conform to ECMAScript 5.1

例外

所有当前的浏览器都暴露了一个类型为 undefined 的非标准宿主对象 document.all。

typeof document.all === 'undefined';

尽管规范允许为非标准的外来对象定制类型标签，但它要求这些类型标签与预定义标签不同。document.all的类型标记为“undefined”的情况必须被列为违反规则的特殊情况。

规范

Specification	Status	Comment
ECMAScript Latest Draft (ECMA-262)The definition of 'The typeof Operator' in that specification.	Living Standard
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'The typeof Operator' in that specification.	Standard
ECMAScript 5.1 (ECMA-262)The definition of 'The typeof Operator' in that specification.	Standard
ECMAScript 3rd Edition (ECMA-262)The definition of 'The typeof Operator' in that specification.	Standard
ECMAScript 1st Edition (ECMA-262)The definition of 'The typeof Operator' in that specification.	Standard	Initial definition. Implemented in JavaScript 1.1.

浏览器兼容性

Feature	Chrome	Edge	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)

Feature	Android	Chrome for Android	Edge	Firefox Mobile (Gecko)	IE Mobile	Opera Mobile	Safari Mobile
Basic support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)