Some small knowledge of the This keyword in JavaScript _javascript tips

JavaScript should be one of the most popular cross-platform languages that have been playing with some interesting things at the front end, and found that the language was not well mastered. A little trifles, so I want to take advantage of this time when there are free to add a little missing things.

The implicit binding of this

At first it was something I was confused about, and I didn't understand it when I first saw it. Then, in similar cases, the same problem can be solved in a similar way. Then try to clarify this knowledge, easy to find.

This is a design error in a JavaScript language, but it seems that the error is unavoidable, the function is an object, the array is an object, and so on. Citing the example in the Javascript:the good Parts

Copy Code code as follows:

function Add (a,b) {return a+b}
var sum = Add (3,4);
Console.log (sum); sum = 7

At this point the result of sum is 7.

Copy Code code as follows:

> typeof add
> ' Number '

As you can see here, the type of add is numeric.

When calling a function in this mode, this is bound to a global variable.
That is, in the current environment, we can call this

Copy Code code as follows:

This.add (3,4)

This is the implicit binding of this, and this is bound in a different way.

Copy Code code as follows:

var hello = function () {
Return "Hello," + this.name;
};
Name = ' this ';
Console.log (hello ());

Then we'll get hello,this. And when

Copy Code code as follows:

var hello = function () {
Return "Hello," + this.name;
};
var user = {
Hello:hello,
Name: ' Phodal ',

};
Console.log (User.hello ());

Then hello in user points to the Hello function, which, in our understanding, is not possible, so it is a bug.

If we define a variable in this method and assign it a value of this, then the internal function can access this by that variable.

var that = this

So when things get a little bit more complicated, we need to use:

Copy Code code as follows:

VAT that = this;

Tips

The scope of a 1.this variable is always determined by its closest enclosing function.
2. Use a local variable (such as Me,self,that) to make the this binding available internally.

A simple example:

Copy Code code as follows:

var M = function () {
THIS.name = "M";
};

var MM = function () {
z = new M ();
THIS.name = "MM";

Z.printname = function () {
Console.log (this.name);
};
return Z.printname ();
};

var mm = new mm;

This point is the M function, and by MM itself. If we remove this from the M, then the return is a undefined. So we create an alias for the current this scope, such as that or self, and so on:

Copy Code code as follows:

var MM = function () {
z = new M ();
THIS.name = "MM";
var self = this;
Z.printname = function () {
Console.log (Self.name);
};
return Z.printname ();
};

So you can return a mm. In addition, the bind method of the callback function can be used in ES5.

Copy Code code as follows:

var MM = function () {
z = new M ();
THIS.name = "MM";
Z.printname = function () {
Console.log (this.name);
}.bind (this);
return Z.printname ();
};

Bind can bind a method to the receiver.

Other

Another hello,world.

Encountered print (' Hello ') (' World ') in an accidental opportunity, and then output ' Hello, world '.

The so-called higher-order function, which seems to be very useful, is interested in looking at the next article.