JS中类型判断

基本类型

• undefined
• null
• Boolean
• Number
• String
• Object
• Symbol(ES6)

常见的引用类型

• function
• Array
• set
• map
  …

一般来说有三种检测类型的方法：typeof 、instanceof 和 Object.proprety.toString.call()

typeof

//typeof
console.log("typeof:")
console.log(typeof(undefined)); //undefined
console.log(typeof(null)); //object
console.log(typeof(false)); //boolean
console.log(typeof(123)); //number
console.log(typeof("123")); //string
console.log(typeof({})); //object
console.log(typeof(Symbol(1)));//symbol
console.log(typeof([])); //object

instanceof

console.log("instanceof:")
// console.log(undefined instanceof undefined); //报错
// console.log(null instanceof null); // 报错
console.log(false instanceof Boolean);
console.log(new Boolean(false) instanceof Boolean);
console.log(123 instanceof Number);
console.log(new Number(123) instanceof Number);
console.log("123" instanceof String);
console.log(new String("123") instanceof String);
console.log({} instanceof Object);
console.log(Symbol(1) instanceof Symbol);
console.log([] instanceof Array);

instanceof的原理是查找对象中的原型链上是否有第二个参数的构造函数，如果有则返回true，反之false。

Object.proprety.toString.call()

console.log("Object.property.toString.call():")
//Object.property.toString.call()
console.log(Object.prototype.toString.call(undefined)) //报错
console.log(Object.prototype.toString.call(null)) //报错
console.log(Object.prototype.toString.call(false))
console.log( Object.prototype.toString.call(123))
console.log(Object.prototype.toString.call("123"))
console.log(Object.prototype.toString.call({}));
console.log(Object.prototype.toString.call(Symbol(1)));
console.log(Object.prototype.toString.call([]));

在检测基本类型如，number、boolean时会将其转为对应的引用类型。

参考

红宝书
https://blog.youkuaiyun.com/zjy_android_blog/article/details/81023177