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1、typeof 类型判断

缺点：无法区分null 、对象、数组，Map，Set，WeakMap，WeakSet、RegExp等

注意：通过构造函数创建的变量typeof 后是都是object

var a = ''
console.log(typeof a)  // string

a = 0
console.log(typeof a)  // number

a = null
console.log(typeof a)  // object

a = undefined
console.log(typeof a)  // undefined

a = true
console.log(typeof a)  // boolean

a = Symbol()
console.log(typeof a)  // symbol

a = {}
console.log(typeof a)  // object

a = []
console.log(typeof a)  // object

a = function(){}
console.log(typeof a)  // function

a = /\d+/
console.log(typeof a)  // object

a = new Date()
console.log(typeof a)  // object

a = new Map()
console.log(typeof a)  // object

a = new Set()
console.log(typeof a)  // object

a = new WeakMap()
console.log(typeof a)  // object

a = new WeakSet()
console.log(typeof a)  // object

2、instanceof 判断

缺点：

    1、只能判断通过构造函数创建的变量，字面量创建的变量无法判断

    2、只能区分对象是否存在目标对象的原型链上

var a = ''
console.log(a instanceof String)  // false

a = new String('')
console.log(a instanceof String)  // true

// Vue2.6源码，使用 instanceof
function Vue(options){
    if(this instanceof Vue){
    }
}

a = new Vue({})
console.log(a instanceof Vue) // true

3、通过构造函数的名称判断

缺点：

    1、不能判断null和undefined

    2、构造函数的指向是可以改变的，因此会存在类型判断不准确

var a = ''
console.log(a.constructor) // ƒ String() { [native code] }

a = new String('')
console.log(a.constructor) // ƒ String() { [native code] }
console.log(a.constructor.name)  // String

a = 1
console.log(a.constructor.name) // Number

a = false
console.log(a.constructor.name) // Boolean

a = []
console.log(a.constructor.name) // Array

a = {}
console.log(a.constructor.name) // Object

a = new Date()
console.log(a.constructor.name) // Date

a = function(){}
console.log(a.constructor.name) // Function

a = /\d+/
console.log(a.constructor.name) // RegExp

a = Symbol()
console.log(a.constructor.name) // Symbol

a = new Map()
console.log(a.constructor.name) // Map

a = new Set()
console.log(a.constructor.name) // Set

a = new WeakMap()
console.log(a.constructor.name) // WeakMap

a = new WeakSet()
console.log(a.constructor.name) // WeakSet

4、toString方法判断

优点：能够校验所有的数据类型

缺点：不能判断谁是谁的实例

var toString = Object.prototype.toString

var a = null
console.log(toString.call(a))  // [object Null]

a = undefined
console.log(toString.call(a))  // [object Undefined]

a = ''
console.log(toString.call(a))  // [object String]

a = 1
console.log(toString.call(a))  // [object Number]

a = false
console.log(toString.call(a))  // [object Boolean]

a = {}
console.log(toString.call(a))  // [object Object]

a = []
console.log(toString.call(a))  // [object Array]

a = function(){}
console.log(toString.call(a))  // [object Function]

a = Symbol()
console.log(toString.call(a))  // [object Symbol]

a = /\d+/
console.log(toString.call(a))  // [object RegExp]

a = new Map()
console.log(toString.call(a))  // [object Map]

a = new Set()
console.log(toString.call(a))  // [object Set]

a = new WeakMap()
console.log(toString.call(a))  // [object WeakMap]

a = new WeakSet()
console.log(toString.call(a))  // [object WeakSet]