JS类型判断全总结

为了不死在笔试，为秋招作的准备

分为两个部分，一部分讲方法，一部分判断实例

1.Array.prototype 是数组,Object.prototype是对象,Function.prototype是函数

设要判断的变量为variable,要判断的类型X

一、方法：

- 1.typeof variable === X

- 2.variable instanceof X

- 3.variable.constructor === X

- 4.Object.prototype.toString.call(variable) === X

（1）typeof

判断基本类型和Object时

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

值得注意的examples:

typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number';

typeof '1' === 'string';
typeof (typeof 1) === 'string';

typeof [1, 2, 4] === 'object';
//new 开头的都是'object'
typeof new Date() === 'object';
typeof new Boolean(true) === 'object';
typeof new Number(1) === 'object';
typeof new String('abc') === 'object';

typeof class C {} === 'function';
typeof Math.sin === 'function';
//几个特列
typeof undefined === 'undefined';
typeof document.all === 'undefined';
typeof null === 'object';
typeof /regularexpression/ === 'object';

（2）instanceof

功能：Checks the current object and returns true if the object

is of the specified object type.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof

https://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript

值得注意的examples:

var color1 = new String("green");
color1 instanceof String; // returns true
var color2 = "coral"; //no type specified
color2 instanceof String; // returns false (color2 is not a String object)

// So what instanceof does is it checks the object to see if it is of the type specified
var c = new Customer();
c instanceof Customer; //Returns true
c instanceof String; //Returns false

（3）variable.constructor === X

>

（4）

1.variable.constructor === Array

2.variable instanceof Array

3.Array.isArray(variable)

4.Object.prototype.toString.call(variable) === ‘[object Array]’;