Cet article JavaScript didacticiel vous des instructions compl?tes rempli et des exemples de code JavaScript pour utiliser cet objet JavaScript Vous pouvez apprendre beaucoup de connaissances de base JavaScript , comme la port?e , d'appeler une fonction/m?thode , etc
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JavaScript is a great programming language. That would have been a controversial statement a few years ago, but developers have rediscovered its beauty and elegance. If you dislike JavaScript, it’s probably because:

• You’ve encountered browser API differences or problems — which isn’t really JavaScript’s fault.
• You’re comparing it to a class-based language such as C++, C# or Java — and JavaScript doesn’t behave in the way you expect.

One of the most confusing concepts is the ‘this’ keyword. In most languages, ‘this’ is a reference to the current object instantiated by the class. In JavaScript, ‘this’ normally refers to the object which ‘owns’ the method, but it depends on how a function is called.

Global Scope

If there’s no current object, ‘this’ refers to the global object. In a web browser, that’s ‘window’ — the top-level object which represents the document, location, history and a few other useful properties and methods.

window.WhoAmI = "I'm the window object";
alert(window.WhoAmI);
alert(this.WhoAmI); // I'm the window object
alert(window === this); // true

Calling a Function

‘this’ remains the global object if you’re calling a function:

window.WhoAmI = "I'm the window object";
function TestThis() {
	alert(this.WhoAmI); // I'm the window object
	alert(window === this); // true
}
TestThis();

Calling Object Methods

When calling an object constructor or any of its methods, ‘this’ refers to the instance of the object — much like any class-based language:

window.WhoAmI = "I'm the window object";
function Test() {
	this.WhoAmI = "I'm the Test object";
	this.Check1 = function() {
		alert(this.WhoAmI); // I'm the Test object
	};
}
Test.prototype.Check2 = function() {
	alert(this.WhoAmI); // I'm the Test object
};
var t = new Test();
t.Check1();
t.Check2();

Using Call or Apply

In essence, call and apply run JavaScript functions as if they were methods of another object. A simple example demonstrates it further:

function SetType(type) {
	this.WhoAmI = "I'm the "+type+" object";
}
var newObject = {};
SetType.call(newObject, "newObject");
alert(newObject.WhoAmI); // I'm the newObject object
var new2 = {};
SetType.apply(new2, ["new2"]);
alert(new2.WhoAmI); // I'm the new2 object

The only difference is that ‘call’ expects a discrete number of parameters while ‘apply’ can be passed an array of parameters.

That’s ‘this’ in a nutshell! However, there are several gotchas which may catch you out. We’ll discuss those in my next chapter

In above chapter, we looked at JavaScript�s this statement and how it can change depending on the context of the function call. Today, we�ll examine several situations where this could catch you out�

1. Forgetting �new�

Consider the following code:

window.WhoAmI = "I'm the window object";
function Test() {
  this.WhoAmI = "I'm the Test object";
}
var t = Test();
alert(window.WhoAmI); // I'm the Test object
alert(t.WhoAmI); // t is undefined

What we really meant is:

var t = new Test();

The omission of the new statement gave us undesirable results. Other languages would throw an error when faced with a direct call to a constructor but JavaScript simply treats it like any other function call. this is taken to be the global window object and no value is returned from Test() so t becomes undefined.

This situation can be fixed if you�re writing a JavaScript library for third-party developers. Refer to Fixing Object Instances in JavaScript.

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