Detailed explanation of application and call usage examples of javascript changing the internal point of the function body

call and apply both exist to change the context that is, the context when a certain function is running. In other words, to Change the pointer of this inside the function body.

call and apply have exactly the same function, but the way they accept parameters is different.

Method definition
apply
Function.apply(obj,args) method can receive two parameters:

obj: This object will replace this object in the Function class

args: This is an array or array-like, and the apply method takes the elements in this collection Passed as a parameter to the called function.

call

The first parameter of the call method is the same as the first parameter of the apply method , except The second parameter is a parameter list

In non-strict mode, when the first parameter is passed as null or undefined, this in the function body will point to the default host object. In the browser It is window

var test = function(){
  console.log(this===window);
}
test.apply(null);//true
test.call(undefined);//true

Usage

"Hijacking" method of others

At this time, the logName method in foo will be referenced by bar, this points to bar

var foo = {
  name:"mingming",
  logName:function(){
    console.log(this.name);
  }
}
var bar={
  name:"xiaowang"
};
foo.logName.call(bar);//xiaowang

Implementing inheritance

##

function Animal(name){   
  this.name = name;   
  this.showName = function(){   
    console.log(this.name);   
  }   
}   

function Cat(name){  
  Animal.call(this, name);  
}   

var cat = new Cat("Black Cat");   
cat.showName(); //Black Cat

In actual development, we often encounter this pointer A scene that was inadvertently changed.

There is a local
fun method. When fun is called as a normal function, fun internal this points to window, but we often want it to point to the #test node, see the following code:

window.id="window";
document.querySelector('#test').onclick = function(){
  console.log(this.id);//test
  var fun = function(){
    console.log(this.id);
  }
  fun();//window
}

Use

call ,applyWe can easily solve this problem

window.id="window";
document.querySelector('#test').onclick = function(){
  console.log(this.id);//test
  var fun = function(){
    console.log(this.id);
  }
  fun.call(this);//test
}

Of course you can also do this, but in

ECMAScript 5# In ##strict mode, this in this case has been stipulated not to point to the global object, but to undefined:

##

window.id="window";
document.querySelector('#test').onclick = function(){
  var that = this;
  console.log(this.id);//test
  var fun = function(){
    console.log(that.id);
  }
  fun();//test
}

function func(){
  "use strict"
  alert ( this );  // 输出：undefined
}
func();

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