JavaScript Function Invocation
THE WORLD'S LARGEST WEB DEVELOPER SITE
×

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Scope JS Events JS Strings JS String Methods JS Numbers JS Number Methods JS Arrays JS Array Methods JS Array Sort JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS Conditions JS Switch JS Loop For JS Loop While JS Break JS Type Conversion JS Bitwise JS RegExp JS Errors JS Debugging JS Hoisting JS Strict Mode JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words JS Versions JS JSON

JS Forms

JS Forms Forms API

JS Objects

Object Definitions Object Properties Object Methods Object Constructors Object Prototypes

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Closures

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Objects JSON Arrays JSON Parse JSON Stringify JSON PHP JSON HTML JSON JSONP

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Quiz JS Certificate

JS References

JavaScript Objects HTML DOM Objects


JavaScript Function Invocation


The code inside a JavaScript function will execute when "something" invokes it.


Invoking a JavaScript Function

The code inside a function is not executed when the function is defined.

The code inside a function is executed when the function is invoked.

It is common to use the term "call a function" instead of "invoke a function".

It is also 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.


Invoking a Function as a Function

Example

function myFunction(a, b) {
    return a * b;
}
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);    // Will also return 20
Try it Yourself »

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



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.


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

var x = myFunction();            // x will be the window object

function myFunction() {
   return this;
}
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.


Invoking a Function as a Method

In JavaScript you can define functions 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.


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.