如何利用js语法判断数据类型

typeof方法

typeof 123  // 返回 "number"
typeof "Hello World"  // 返回 "string"
typeof true  // 返回 "boolean"
typeof undefined  // 返回 "undefined"
typeof null  // 返回 "object"
typeof [1, 2, 3]  // 返回 "object"
typeof {"name": "小米", "age": 32}  // 返回 "object"
typeof function() {}  // 返回 "function"

Object.prototype.toString方法

Object.prototype.toString.call(123)  // 返回 "[object Number]"
Object.prototype.toString.call("Hello World")  // 返回 "[object String]"
Object.prototype.toString.call(true)  // 返回 "[object Boolean]"
Object.prototype.toString.call(undefined)  // 返回 "[object Undefined]"
Object.prototype.toString.call(null)  // 返回 "[object Null]"
Object.prototype.toString.call([1, 2, 3])  // 返回 "[object Array]"
Object.prototype.toString.call({"name": "小米", "age": 32})  // 返回 "[object Object]"
Object.prototype.toString.call(function() {})  // 返回 "[object Function]"

instanceof方法

var arr = [1, 2, 3];
console.log(arr instanceof Array);  // 返回 true
 
var obj = {"name": "小米", "age": 22};
console.log(obj instanceof Object);  // 返回 true
 
function Person(name, age) {
  this.name = name;
  this.age = age;
}
var alex = new Person("小米", 32);
console.log(alex instanceof Person);  // 返回 true

constructor方法

var arr = [1, 2, 3];
console.log(arr.constructor === Array);  // 返回 true
 
var obj = {"name": "小米", "age": 18};
console.log(obj.constructor === Object);  // 返回 true
 
function Person(name, age) {
  this.name = name;
  this.age = age;
}
var alex = new Person("小米", 18);
console.log(alex.constructor === Person);  // 返回 true