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JavaScript this Object: Tutorials and Examples This JavaScript article tutorial guides you a full completed instructions and JavaScript code examples to use this JavaScript object. You can learn many basic JavaScript knowledges such as scope, call a function/method, etc.

Please go to the full post page for all details or read other related JavaScript tutorials:
- JavaScript Objects: Compare and Clone
- Let Get Started Object-Oriented Programming JavaScript
- Object JavaScript Test Quiz
- Simple Concepts about Types and Objects in JavaScript OOP
- Object Oriented JavaScript Techniques


Label: JavaScript this Object, this JavaScript object, JavaScript knowledge

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2. Module madness

This one will give you a headache. Examine the following code which uses a module pattern:



window.WhoAmI = "I'm the window object";
var Module = function() {
  this.WhoAmI = "I'm the Module object";
  function Test() {
    this.WhoAmI = "I'm still the Module object";
  }
  return {
    WhoAmI: WhoAmI,
    Test: Test
  };
}();
alert(Module.WhoAmI); // I'm the Module object
alert(window.WhoAmI); // I'm the Module object
Module.Test();
alert(Module.WhoAmI); // I'm still the Module object

The code looks logical ó so why is window.WhoAmI saying itís the module object?

We need to remember that we have a self-executing function. Itís results are returned to the Module variable but, when itís first run, Module doesnít exist. this is therefore the global window object. In other words, this.WhoAmI = window.WhoAmI = "I'm the Module object".

The function returns a JavaScript object with a WhoAmI property with a value of 'WhoAmI'. But what does that refer to? In this case, the JavaScript interpreter works up its prototype chain until it magically finds window.WhoAmI ("I'm the Module object").

Finally, we run the Test() method. However, Module has now been created so, within the Test function, this refers to the Module object so it can correctly set the WhoAmI property.

In summary, avoid using this within a module to refer to the module itself! You should never need it.

3. Method misconceptions

Hereís another JavaScript pattern which will screw with your synapses:



var myObject = {};
myObject.method = function() {
  this.WhoAmI = "I'm myObject.method";
  function Test() {
    this.WhoAmI = "I'm myObject.method.Test()";
  }
  Test();
  return this.WhoAmI;
};
alert(myObject.method()); // I'm myObject.method

In this example, Test() is a private function executed within myObject.method(). At first glance, you would expect this within Test() to reference myObject. It doesnít: it refers to the global window object since itís just another function.

If you want to reference myObject within the private function, youíll require a closure, for example:



var myObject = {};
myObject.method = function() {
  this.WhoAmI = "I'm myObject.method";
  var T = this;
  function Test() {
    T.WhoAmI = "I'm myObject.method.Test()";
  }
  Test();
  return this.WhoAmI;
};
alert(myObject.method()); // I'm myObject.method.Test()

4. Referencing methods

Hereís a little code which, fortunately, will work exactly as you expect:



var myObject = {};
myObject.WhoAmI = "I'm myObject";
myObject.method = function() {
  this.WhoAmI = "I'm myObject.method";
};
// examine properties
alert(myObject.WhoAmI); // I'm myObject
myObject.method();
alert(myObject.WhoAmI); // I'm myObject.method

Letís make a minor change and assign myObject.method to another variable:



// examine properties
alert(myObject.WhoAmI); // I'm myObject
var test = myObject.method;
test();
alert(myObject.WhoAmI); // I'm myObject

Why hasnít myObject.WhoAmI changed? In this case, the call to test() acts like a regular function call so this refers to the window object rather than myObject.

If you think thatís nasty, wait until we take a look at JavaScript event handlers in my next post!

JavaScript by day


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