javascript create function

Question

What is the difference between this:

var doSomething=function()
{
 //do blah blah blah... 
}

And this:

function doSomething()
{
 //do blah blah blah... 
}

Another question: In PHP, we create a function by doing this:

function doSomething(a,b)
{
 //do something to variable a and b...
}

In JavaScript, we may have an object before the function:

object.doSomething(a);

My second question is, how would you create a function which requires an object in JavaScript?

• Kris Ivanov: difference is that second function is defined at parse-time for a script block, whereas the first function is defined at run-time
• Malachi: possible duplicate of var functionName = function() {} vs function functionName() {}

Answer 1

The number one Google result for "function statement vs expression javascript" is another Stack Overflow question:

What is the difference between a function expression vs declaration in JavaScript?

It references the following article, which is the definitive reference on the subject:

http://kangax.github.com/nfe/

• rsalmeidafl: For questions related to the javascript language itself, I found the book "JavaScript: The Definitive Guide", By David Flanagan - really helpful. However, for this particular question, I believe the links provided are an even better source =)

Answer 2

The difference between var fun = function() {} and function fun() {} is that in the first case it is stored in the variable fun. To call the function, you have to call fun(). Having it in a variable lets you pass the function around.

You can create objects by using functions

function MyClass() {
 this.fun = function() { alert('Hello world'); }
}
var obj = new MyClass();
obj.fun();

or JSON

var obj = {
 fun: function() { alert('Hello world'); }
};
obj.fun();

You can further extend the objects or their prototypes with new functions.

Edit. Sorry for the wrong answer: one shouldn't try to do these kinds of things at 4 am.

• Praneeth: @Aleksi: Even for the second function you need to call fun(); at some point.
• Jeff: In neither case is the function executed immediately. To invoke the function immediately it would have to be var fun = function(){}(); or function fun() {}()
• Wayne: The body of the function is not executed immediately in either case.
• Aleksi Yrttiaho: You are absolute right, my mistake. Both are definitions, not calls. The first one just lets you use a functions as a variable and pass it around. Upvotes allaround for rapid corrections!
• david: Your first example is completely wrong, you're setting 'fun' on the window object and then setting obj to undefined.

Answer 3

One question at a time.

To answer your first question, there is not a huge difference.

function doSomething() {} 

is technically equivalent to:

var doSomething;
doSomething = function() {};

technically what happens in this case is the variable declaration gets hoisted to the top of your script.

• david: They're not quite equivalent, the first one hoists the definition as well as the declaration, the second only hoists the declaration.

Answer 4

For the second part of the question, we just do something like

object.doSomething = function(a) { ... }

which is one reason the function literal is so useful.