//全局对象用于默认绑定function foo(){ ???console.log(this.a)}var a=2;foo(); ?//2

//严格模式下，不能将全局对象用于默认绑定function foo(){ ???‘use strict‘; ???console.log(this.a)}var a=2;foo(); ?//TypeError: this is undefined

//在严格模式下调用foo()则不影响默认绑定function foo(){ ???console.log(this.a)}var a=2;(function(){ ???‘use strict‘; ???foo();})(); ?//2

//当函数拥有上下文对象时，隐式绑定规则会把函数调用中的this绑定到这个上下文对象function foo(){ ???console.log(this.a)}var obj={ ???a:2, ???foo:foo}obj.foo(); ???//2

//对象属性链中只有上一层或者最后一层在调用位置中起作用function foo(){ ???console.log(this.a)}var obj2={ ???a:42, ???foo:foo}var obj1={ ???a:2, ???obj2:obj2}obj1.obj2.foo(); ???//42

//this 被隐式绑定的函数会丢失绑定对象，也就是说会应用默认绑定，从而把this绑定到全局对象或者undefined上function foo(){ ???console.log(this.a)}var obj={ ???a:2, ???foo:foo} var bar=obj.foo;var a="hello";bar(); ?//hello

//参数传递其实就是隐式传递，因此我们传入函数是也会被隐式赋值function foo(){ ???console.log(this.a)}function doFoo(fn){ ???fn();}var obj={ ???a:2, ???foo:foo}var bar=obj.foo;var a="hello";doFoo(obj.foo) ?//hello

//回调函数丢失this绑定function foo(){ ???console.log(this.a)}var obj={ ???a:2, ???foo:foo} var a="hello";setTimeout(obj.foo,100) ??//hello

//通过foo.call(..),在调用foo时强制把它的this绑定到obj上function foo(){ ???console.log(this.a)}var obj={ ???a:2}foo.call(obj) //2

//硬绑定function foo(){ ???console.log(this.a)}var obj={ ???a:2} var bar=function(){ ???foo.call(obj)};bar(); //2setTimeout(bar,100); ?//2//硬绑定的bar ???不可能再修改它的thisbar.call(window); //2

//硬绑定典型应用场景：创建一个包裹函数，负责接收参数并返回值function foo(something){ ???console.log(this.a,something); ???return this.a + something;}var obj={ ???a:2}var bar=function(){ ???return foo.apply(obj,arguments)}var b=bar(3); ?//2 3console.log(b) ???//5

//硬绑定典型应用场景：创建一个可以重复使用的辅助函数function foo(something){ ???console.log(this.a,something); ???return this.a + something;} ???//简单的辅助绑定函数function bind(fn,obj){ ???return function(){ ???????return fn.apply(obj,arguments) ???}}var obj={ ???a:2}var bar=bind(foo,obj);var b=bar(3); //2 3console.log(b) ?//5