# 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
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# instanceof
通过原型链来判断的
function People() { }
let 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)
let t = new Teacher()
console.log(t instanceof Teacher) // true
console.log(t instanceof People) // true
console.log(t instanceof Student) // false
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# myInstanceof
function myInstanceof(target, origin) {
let 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)
}
}
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