function test() {
 console.log(this);
};

test();
指向window 

嚴(yán)格模式
'use strict';
function test() {
 console.log(this);
};
test();
// undefined

var obj = {
 name: 'segmentFault',
 foo: function() {
 console.log(this.name);
 }
}
obj.foo();
// 'segmentFault'

function test() {
 console.log(this.name);
}
var obj = {
 name: 'qiutc',
 foo: test
}
obj.foo();
// 'qiutc'

 function a(a,b,c) {
 console.log(this.name);
 console.log(a,b,c)
 }
 const b = {
 name: "segmentFault"
 }
 a.call(b,1,2,3) 
 //

 function a() {
 console.log(this.name);
 }
 const b = {
 name: "segmentFault"
 }
 a.bind(b, 1, 2, 3)

 function a() {
 console.log(this.name);
 }
 const b = {
 name: "segmentFault"
 }
 const c = a.bind(b, 1, 2, 3)
 c()
 //此時(shí)

 Function.prototype.myCall = function (context, ...arg) {
 const fn = Symbol('臨時(shí)屬性')
 context[fn] = this
 context[fn](...arg)
 delete context[fn]
 }

//實(shí)現(xiàn)自己的myApply
 Function.prototype.myApply = function (context, arg) {
 const fn = Symbol('臨時(shí)屬性')
 context[fn] = this
 context[fn](...arg)
 delete context[fn]
 }
 const obj2 = {
 a: 1
 }
 test.myApply(obj2, [2, 3, 4])

 //實(shí)現(xiàn)自己的myBind
 Function.prototype.myBind = function (context, ...firstarg) {
 const that = this
 const bindFn = function (...secoundarg) {
 return that.myCall(context, ...firstarg, ...secoundarg)
 }
 bindFn.prototype = Object.create(that.prototype)
 return bindFn
 }

 var fnbind = test.myBind(obj, 2)
 fnbind(3)

Function.prototype.myBind = function (objThis, ...params) {
 const thisFn = this; // 存儲(chǔ)源函數(shù)以及上方的params(函數(shù)參數(shù))
 // 對(duì)返回的函數(shù) secondParams 二次傳參
 let fToBind = function (...secondParams) {
 console.log('secondParams',secondParams,...secondParams)
 const isNew = this instanceof fToBind // this是否是fToBind的實(shí)例 也就是返回的fToBind是否通過(guò)new調(diào)用
 const context = isNew ? this : Object(objThis) // new調(diào)用就綁定到this上,否則就綁定到傳入的objThis上
 return thisFn.call(context, ...params, ...secondParams); // 用call調(diào)用源函數(shù)綁定this的指向并傳遞參數(shù),返回執(zhí)行結(jié)果
 };
 fToBind.prototype = Object.create(thisFn.prototype); // 復(fù)制源函數(shù)的prototype給fToBind
 return fToBind; // 返回拷貝的函數(shù)
};