浅谈this指向

this指向问题对于初学者是一个比较头疼的事，今天我总结了几点关于this指向的问题，也就是我们所说的执行期上下文

1.默认绑定，没人调用指向window

        var name = 'window'
        function test() {
            console.log(this.name)
        }
        test() //window

2.隐式绑定，谁调用this指向谁

           var name = 'window'
            var obj = {
                name:'obj',
                sayName(){
                    console.log(this.name)
                }
            }
            obj.sayName(); //obj

3.显示绑定，bind,call,apply

在这里我也说一下他们之间的区别，bind返回带有明确this指向的函数，call和apply会立即执行函数并且改变this指向，而call和apply的区别在于传参列表不一样

           var name = 'window'
            var obj = {
                name:'obj',
                sayName(){
                    console.log(this.name)
                }
            }
            obj.sayName() //obj
            var sayName = obj.sayName.bind(window)
            sayName() //window
            sayName.call(obj) //window

3.通过new绑定

        function Person(){
            this.name = name
            console.log(this.name)
        }
        var person = new Person() //obj

优先级比较

显示绑定和隐式绑定

        var name = 'window'
        var obj = {
            name:'obj',
            sayName(){
                console.log(this.name)
            }
        }
        obj.sayName() //obj
        obj.sayName.call(window) //window

可以看出显示绑定>隐式绑定

显示绑定和new绑定

       function Person(name){
           this.name= name
           console.log(this)
       }
       var p = Person.bind(window) 
       var newP1 = new p('newP1') //Person{name:'newPerson'}

将this硬绑定到window上，可通过new绑定还是改变了this指向，所以new绑定>显示绑定

即new绑定>显示绑定>隐式绑定