var info = "This is global info";
var obj = {
 info: 'This is local info',
 getInfo: getInfo
}
function getInfo() {
 console.log(this.info);
}
obj.getInfo() //This is local info

getInfo() //This is global info 当前上下文环境被修改了

var info = "This is global info";
var obj = {
 info: 'This is local info',
 getInfo: function getInfo() {
 console.log(this.info);

 var getInfo2 = function getInfo2() {
  console.log(this.info);
 }
 getInfo2();
 }
}
obj.getInfo();

//This is local info
//This is global info

var info = "This is global info";
var obj = {
 info: 'This is local info',
 getInfo: function getInfo() {
 console.log(this.info);

 var self = this;  //将外层this保存到变量中
 var getInfo2 = function getInfo2() {
  console.log(self.info); //指向外层变量代表的this
 }
 getInfo2();
 }
}
obj.getInfo();

//This is local info
//This is local info

...
var anotherFun = obj.getInfo;
anotherFun.call(obj) //This is local info

function add(num1, num2, num3) {
 return num1 + num2 + num3;
}
add.call(null, 10, 20, 30); //60
add.apply(null, [10, 20, 30]); //60

Function.prototype.bind(context[, argument1, argument2])
//函数柯里化部分
function add(num1 ,num2) {
 return num1 + num2;
}
var anotherFun = window.add.bind(window, 10);
anotherFun(20); //30

...
var anotherFun = obj.getInfo.bind(obj)
anotherFun(); //This is local info

if (!Function.prototype.bind) {
 Function.prototype.bind = function (obj) {
 var self = this;
 return function () {
 self.call(obj);
 }
 }
}

...
Function.prototype.bind = function(obj) {
 var args = Array.prototype.slice.call(arguments, 1); 
 //记录下所有第一次传入的参数

 var self = this;
 return function () {
 self.apply(obj, args.concat(Array.prototype.slice.call(arguments)));
 }
}

function fun(){console.log("function called")}
//直接调用
fun() //function called
//构造调用
var obj = new fun() //function called

var a = 5;
function Fun() {
 this.a = 10;
}
var obj = new Fun();
obj.a //10

var obj1 = {
 info: "this is obj1",
 getInfo: () => console.log(this.info)
}

var obj2 = {
 info: "this is obj2",
 getInfo: () => console.log(this.info)
}

obj1.getInfo() //this is obj1
obj2.getInfo() //this is obj2
obj1.getInfo.call(obj2) //this is obj2
obj2.getInfo.call(obj1) //this is obj1

var obj = {}
function foo(num){
 this.num = num;
}

var setNum = foo.bind(obj);
setNum(10);
obj.num //10

var obj2 = new setNum(20);
obj.num //10
obj2.num //20

if (!Function.prototype.bind) { 
 Function.prototype.bind = function (oThis) { 
 if (typeof this !== "function") { 
 throw new TypeError("Function.prototype.bind - 
 what is trying to be bound is not callable"); 
 }
 var aArgs = Array.prototype.slice.call(arguments, 1),
 fToBind = this, 
 fNOP = function () {}, 
 fBound = function () { 
  return fToBind.apply(this instanceof fNOP ? 
    this : oThis || this, 
   aArgs.concat(Array.prototype.slice.call(arguments))); 
 }; 
 fNOP.prototype = this.prototype; 
 fBound.prototype = new fNOP(); 
 return fBound; 
 };
}

var obj = {
 num: 1,
 getNum: function () {
 return function () {
 //this丢失
 console.log(this.num); 
 //此处的this指向window
 }
 }
}
obj.getNum()(); //undefined

var obj2 = {
 num: 2,
 getNum: function () {
 return () => console.log(this.num); 
 //箭头函数内部绑定外部getNum的this，外部this指向调用的对象
 }
}
obj2.getNum()(); //2

if (!Function.prototype.softBind) {
 Function.prototype.softbind = function (obj) {
 var self = this;
 var args = Array.prototype.slice.call(arguments, 1);
 return function () {
 return self.apply((!this || this === (window || global)) ? 
   obj : this, 
   args.concat(Array.prototype.slice.call(arguments)));
 }
 }
}

var slice = Array.prototype.slice;
slice(arguments);
//error

var qsa = document.querySelectorAll;
qsa(something);
//error

var qsa = document.querySelectorAll.bind(document);
qsa(something);

var slice = Function.prototype.call.bind(Array.prototype.slice);
slice(arguments);