js中typeof用法详细介绍

Posted 2021-01-13 诗和远方-ysk

本文章来介绍在js使用中的一个函数typeof用法, typeof 运算符把类型信息当作字符串返回，包括有大家常有变量类型。

typeof 运算符把类型信息当作字符串返回。typeof 返回值有六种可能： "number," "string," "boolean," "object," "function," 和 "undefined."我们可以使用typeof来获取一个变量是否存在，如if(typeof a!="undefined"){}，而不要去使用if(a)因为如果a不存在（未声明）则会出错，对于Array,Null等特殊对象使用typeof一律返回object，这正是typeof的局限性。

typeof 语法中的圆括号是可选项。

if(document.mylist.length != “undefined” ) {} 这个用法有误.

正确的是 if( typeof(document.mylist.length) != “undefined” ) {}

或 if( !isNaN(document.mylist.length) ) {}

typeof的运算数未定义,返回的就是 “undefined”.

运算数为数字 typeof(x) = “number”

字符串 typeof(x) = “string”

布尔值 typeof(x) = “boolean”

对象,数组和null typeof(x) = “object”

函数 typeof(x) = “function”

typeof 运算符返回一个用来表示表达式的数据类型的字符串。
可能的字符串有：”number”、”string”、”boolean”、”object”、”function” 和 “undefined”。
如：

代码如下

alert(typeof (123));//typeof(123)返回”number” alert(typeof (“123″));//typeof(“123″)返回”string”

 
对于Array,Null等特殊对象使用typeof一律返回object，这正是typeof的局限性。

如果我们希望获取一个对象是否是数组，或判断某个变量是否是某个对象的实例则要选择使用instanceof。instanceof用于判断一个变量是否某个对象的实例，如var a=new Array();alert(a instanceof Array);会返回true，同时alert(a instanceof Object)也会返回true;这是因为Array是object的子类。再如：function test(){};var a=new test();alert(a instanceof test)会返回true。

友情提示

a instanceof Object 得到true并不是因为 Array是Object的子对象，而是因为 Array的prototype属性构造于Object，Array的父级是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 5.1

Exceptions

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.