js 数据类型 & 判断

基本类型：number, string, boolean, undefined, null, symbol
复合类型：对象

判断类型的几种方式

typeof

typeof 1 // number
typeof 's' // string
typeof true // boolean
typeof undefined // undefined
typeof null // object // 重点记: 这是一个 js 的 bug *
typeof NaN // number *
const v1 = {}
const v2 = []
const v3 = function () {}
const v4 = () => {}
const v5 = Symbol('s')
const v6 = new Boolean(true)
const v7 = Number('w')
typeof v1 // object
typeof v2 // object
typeof v3 // function *
typeof v4 // function *
typeof v5 // symbol
typeof v6 // object *
typeof v7 // number *

typeof a // ?
typeof b // ?
var a = 1
let b = '1'

instanceof

1 instanceof Number // false
new Number(1) instanceof Number // true
Number(1) instanceof Number // false
Boolean(true) instanceof Boolean // false
new Boolean(false) instanceof Boolean // true
null instanceof Object // false
[] instanceof Object // true

const fn = function () {}
fn instanceof Object // true

Object.prototype.toString.call()

Object.prototype.toString.call(1) // "[object Number]"
Object.prototype.toString.call("s") // "[object Number]"
Object.prototype.toString.call(true) // "[object Boolean]"
Object.prototype.toString.call(null) // "[object Null]"
Object.prototype.toString.call(undefined) // "[object Undefined]"
Object.prototype.toString.call([]) // "[object Array]"
Object.prototype.toString.call([]) // "[object Array]"
Object.prototype.toString.call(() => {}) // "[object Function]"
Object.prototype.toString.call(new Date) // "[object Date]"
Object.prototype.toString.call(Symbol('S')) // "[object Symbol]"

NaN

typeof NaN // number
const a = NaN
a== a // false
a=== a // false

isNaN(a) // true