javascript - Function Invocation

JavaScript Function Invocation

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JavaScript functions can be invoked in 4 different ways.

Each method differs in how this is initialized.

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The this Keyword

In JavaScript, the thing called this, is the object that "owns" the current code.

The value of this, when used in a function, is the object that "owns" the function.

	Note that this is not a variable. It is a keyword. You cannot change the value of this.

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Invoking a JavaScript Function

You have already learned that the code inside a JavaScript function will execute when "something" invokes it.

The code in a function is not executed when the function is defined. It is executed when the function is invoked.

Some people use the term "call a function" instead of "invoke a function".

It is also quite common to say "call upon a function", "start a function", or "execute a function".

In this tutorial, we will use invoke, because a JavaScript function can be invoked without being called.

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Invoking a Function as a Function

Example

function myFunction(a, b) {
    return a * b;
}
myFunction(10, 2);           // myFunction(10, 2) will return 20

Try it Yourself »

The function above does not belong to any object. But in JavaScript there is always a default global object.

In HTML the default global object is the HTML page itself, so the function above "belongs" to the HTML page.

In a browser the page object is the browser window. The function above automatically becomes a window function.

myFunction() and window.myFunction() is the same function:

Example

function myFunction(a, b) {
    return a * b;
}
window.myFunction(10, 2);    // window.myFunction(10, 2) will also return 20

Try it Yourself »

	This is a common way to invoke a JavaScript function, but not a good practice in computer programming. Global variables, methods, or functions can easily create name conflicts and bugs in the global object.

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The Global Object

When a function is called without an owner object, the value of this becomes the global object.

In a web browser the global object is the browser window.

This example returns the window object as the value of this:

Example

function myFunction() {
    return this;
}
myFunction();                // Will return the window object

Try it Yourself »

	Invoking a function as a global function, causes the value of this to be the global object. Using the window object as a variable can easily crash your program.

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Invoking a Function as a Method

In JavaScript you can define function as object methods.

The following example creates an object (myObject), with two properties (firstName and lastName), and a method (fullName):

Example

var myObject = {
    firstName:"John",
    lastName: "Doe",
    fullName: function () {
        return this.firstName + " " + this.lastName;
    }
}
myObject.fullName();         // Will return "John Doe"

Try it Yourself »

The fullName method is a function. The function belongs to the object. myObject is the owner of the function.

The thing called this, is the object that "owns" the JavaScript code. In this case the value of this is myObject.

Test it! Change the fullName method to return the value of this:

Example

var myObject = {
    firstName:"John",
    lastName: "Doe",
    fullName: function () {
        return this;
    }
}
myObject.fullName();          // Will return [object Object] (the owner object)

Try it Yourself »

	Invoking a function as an object method, causes the value of this to be the object itself.

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Invoking a Function with a Function Constructor

If a function invocation is preceded with the new keyword, it is a constructor invocation.

It looks like you create a new function, but since JavaScript functions are objects you actually create a new object:

Example

// This is a function constructor:
function myFunction(arg1, arg2) {
    this.firstName = arg1;
    this.lastName  = arg2;
}

// This creates a new object
var x = new myFunction("John","Doe");
x.firstName;                             // Will return "John"

Try it Yourself »

A constructor invocation creates a new object. The new object inherits the properties and methods from its constructor.

	The this keyword in the constructor does not have a value. The value of this will be the new object created when the function is invoked.

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Invoking a Function with a Function Method

In JavaScript, functions are objects. JavaScript functions have properties and methods.

call() and apply() are predefined JavaScript function methods. Both methods can be used to invoke a function, and both methods must have the owner object as first parameter.

Example

function myFunction(a, b) {
    return a * b;
}
myObject = myFunction.call(myObject, 10, 2);     // Will return 20

Try it Yourself »

Example

function myFunction(a, b) {
    return a * b;
}
myArray = [10, 2];
myFunction.apply(myObject, myArray);             // Will also return 20

Try it Yourself »

Both methods takes an owner object as the first argument. The only difference is that call() takes the function arguments separately, and apply() takes the function arguments in an array.

In JavaScript strict mode, the first argument becomes the value of this in the invoked function, even if the argument is not an object.

In "non-strict" mode, if the value of the first argument is null or undefined, it is replaced with the global object.

	With call() or apply() you can set the value of this, and invoke a function as a new method of an existing object.

source - http://www.w3schools.com/js/js_function_invocation.asp