js 如何判断变量类型

自定义一些类型

const boolean = true
const string = '这是一个字符串'
const number = 1
const array = []
const object = { name: '张三' }
const functionType = function () {}
const nan = NaN
const undefinedType = undefined
const nullType = null
const Reg = new RegExp()
const set = new Set()
const map = new Map()
const date = new Date()
const math = Math

1、typeof

console.log(typeof boolean);            // boolean
console.log(typeof string);             // string
console.log(typeof number);             // number
console.log(typeof array);              // object
console.log(typeof object);             // object
console.log(typeof functionType);       // function
console.log(typeof nan);                // number
console.log(typeof undefinedType);      // undefined
console.log(typeof nullType);           // object
console.log(typeof Reg);                // object
console.log(typeof set);                // object
console.log(typeof map);                // object
console.log(typeof date);               // object
console.log(typeof math);               // object
console.log(typeof window);             // object
console.log(typeof document);           // object

缺点：typeof只能识别boolean、string、number、object、function

2、instanceof

console.log(boolean instanceof Boolean);            // false
console.log(string instanceof String);              // false
console.log(number instanceof Number);              // false
console.log(array instanceof Array);                // true
console.log(object instanceof Object);              // true
console.log(functionType instanceof Function);      // true
console.log(nan instanceof Number);                 // false

缺点：instanceof不能能识别基本类型和null、undefined

3、constructor

缺点：constructor的指向可能会被改变，且null，和undefined 没有constructor

4、Obeject.prototype.toString.call()

console.log(Object.prototype.toString.call(boolean));                // [object Boolean]
console.log(Object.prototype.toString.call(string));                 // [object String]
console.log(Object.prototype.toString.call(number));                 // [object Number]
console.log(Object.prototype.toString.call(array));                  // [object Array]
console.log(Object.prototype.toString.call(object));                 // [object Object]
console.log(Object.prototype.toString.call(functionType));           // [object Function]
console.log(Object.prototype.toString.call(nan));                    // [object Number]
console.log(Object.prototype.toString.call(undefinedType));          // [object Undefined]
console.log(Object.prototype.toString.call(nullType));               // [object Null]
console.log(Object.prototype.toString.call(Reg));                    // [object RegExp]
console.log(Object.prototype.toString.call(set));                    // [object Set]
console.log(Object.prototype.toString.call(map));                    // [object Map]
console.log(Object.prototype.toString.call(date));                   // [object Date]
console.log(Object.prototype.toString.call(math));                   // [object Math]
console.log(Object.prototype.toString.call(window));                 // [object Window]
console.log(Object.prototype.toString.call(document));               // [object HTMLDocument]

大部分类型还是可以识别的

缺点：Obeject.prototype.toString.call()不能能识别非原生构造函数的构造函数名