The problem of this direction in javasript and the method of changing this direction

Wahaha a 2020-11-09 22:18:56
problem direction javasript method changing


I'm learning javascript We tend to be this The direction of the problem made dizzy turn , Let's learn today this Direction problem of , And change this The way to point .

One .this Direction problem of

I'm learning this We need to understand two things before we point to the problem :

1:this Always point to an object ;
2:this The direction of the function depends entirely on the location of the function call ;

We can understand the first point , Because in javascript Everything in is the object . The second point is actually easy to understand , When the position of function call is different ,this It's not the same thing , So you could say this Can be changed dynamically , Let's start with a simple example this The direction of is changing

<script>
function fun(){
console.log(this.name);
}
var change={
name:'hello',
f:fun
}
var name ='world'
var result=change.f()//hello
fun();//world
</script>

Through the above example, we can see clearly that this Changes in the direction of , Because there's a function in the object change Inside , therefore this It's the object outside the function that points to , So output hello.

After reading the above examples, we will have a good understanding of this Dynamic point to switch has a certain understanding of .

 

So next , We are right. this Make a summary of the most frequently used situations , The most common basic is the following 3 Kind of :

Methods in objects , Event binding , Constructors , Timer

There's no need to talk about the first two , Let's take a look at the timer this Point to the problem ,

var obj = {
fun:function(){
this ;
}
}
​
setInterval(obj.fun,1000); // this Point to window object 
setInterval('obj.fun()',1000); // this Point to obj object 

setInterval()  yes window Object , Take two parameters , The first parameter is allowed to be a function or an executable section JS Code , The second parameter is the interval between the execution of the previous function or code ;

In the code above ,setInterval(obj.fun,1000)  The first parameter of obj Object's fun , because JS Functions in can be passed by reference as values , In fact, the address of this function is passed as an argument to  setInterval  Method , In other words  setInterval  The first parameter of takes a function , So at this time 1000 Milliseconds later , The function is already running in window Under the object , That is, the caller of the function has become window object , So one of them this It points to the overall situation window object ;

And in the  setInterval('obj.fun()',1000)  First parameter in , Actually, it's the incoming executable segment JS Code ;1000 Millisecond later when JS When the engine comes to execute this code , It is through  obj  Object to find  fun  Function and call to execute , So the environment of the function is still object  obj  Inside , So inside the function this It points to  obj  object ;

In addition to these, we need to understand three things that can change this Function pointed to , Including the arrow function ,call(),apply()

Arrow function : There's an official explanation , One of the reasons for the introduction of the arrow function , It's not bound this; In the arrow function , Arrowhead function this Set as a closed lexical environment , let me put it another way , In the arrow function this Depending on the environment in which the function was created .

var globalObject = this;
var foo = (() => this);
console.log(foo() === globalObject); // true

// And then the code above
// Call as a method of an object
var obj = {foo: foo};
console.log(obj.foo() === globalObject); // true
// Try to use call To set this
console.log(foo.call(obj) === globalObject); // true
// Try to use bind To set this
foo = foo.bind(obj);
console.log(foo() === globalObject); // true

in any case ,foo  Of  this  Is set to the environment in which it was created ( In the example above , It's the global object ). The same applies to arrow functions created within other functions : These arrow functions are this Set as a closed lexical environment .

// Create a containing bar Methodical obj object ,
// bar Returns a function ,
// This function returns this,
// The returned function is created with the arrow function ,
// So it's this Is permanently bound to its outer function this.
// bar The value of can be set in the call , This in turn sets the value of the return function .
var obj = {
bar: function() {
var x = (() => this);
return x;
}
};
// As obj Object to call bar, It's this Bound to the obj.
// Assign the reference of the returned function to fn.
var fn = obj.bar();
// Call directly fn It is not set this,
// Usually ( That is, without using the arrow function ) Default to global object 
// In strict mode, it is undefined
console.log(fn() === obj); // true
// But notice , If you just quote obj Methods ,
// Instead of calling it 
var fn2 = obj.bar;
// So after calling the arrow function ,this Point to window, Because it comes from bar Inherited this.
console.log(fn2()() == window); // true
call and apply Method : Take an object as call perhaps apply The first parameter of ,this Will be bound to this parameter object 
var obj = {parent:' male '};
var parent = '28';
function child(obj){
console.log(this.parent);
}
child(); // 28 
child.call(obj); // male 
child.apply(obj); // male 

 

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