typeof 是否能正确判断类型？instanceof 能正确判断对象的原理是什么？

typeof

• typeof对于原始值类型除了null外都可以准确判断
• typeof 对于对象来说，除了函数都会显示 object

所以说 typeof 并不能准确判断变量到底是什么类型

typeof 1 // number
typeof '1' // string
typeof true // boolean
typeof undefined // undefined
typeof Symbol // symbol
typeof null // object(实际上null不是Object)
typeof [] // object
typeof {} // object
typeof Function // function

instanceof

通过原型链来判断的

function People() { }
const p = new People()
console.log(p instanceof People) // true

function Teacher() { }
Teacher.prototype = Object.create(People.prototype)
function Student() { }
Student.prototype = Object.create(People.prototype)

const t = new Teacher()
console.log(t instanceof Teacher) // true
console.log(t instanceof People) // true
console.log(t instanceof Student) // false

myInstanceof

function myInstanceof(target, origin) {
  const prototype = origin.prototype
  target = Object.getPrototypeOf(target)
  while (true) {
    if (target === null || target === undefined) {
      return false
    }
    if (target === prototype) {
      return true
    }
    target = Object.getPrototypeOf(target)
  }
}