3.原型链

var arr=[1,2,3]
console.log(arr.__proto__);//数组的所有方法
console.log(arr.__proto__ == Array.prototype);//true
console.log(arr.__proto__.__proto__)//;object.prototype
console.log(arr.__proto__.__proto__ == Object.prototype)//true
console.log(arr.valueOf())//[1, 2, 3]