Question

Possible Duplicate:
JavaScript: var functionName = function() {} vs function functionName() {}

What is the difference between those two functions

function a()
{
    b=2;
    alert(b);
}
a(); 

and this function

var a=function()
{
    b=2
    alert(b);
}
a();

what is the main difference

Answer 1

The main difference is that when you declare function:

function a(){
    // something...
}

it becomes accessible in the same scope even before the place in the code where it is declared.

But when you assign anonymous function to a variable:

var a = function(){
    // something...
};

it is not available before the assignement.

When the functions are created

It is a result of when the function is actually created. In first case it is created when the code is compiled, while in the second case the function is created when the interpreter reaches the line of assignment.

Test code

You can see the difference I mentioned above by executing the following code (jsfiddle):

try {
    a();
} catch(e) {
    alert('problem calling function a(): ' + e);
};
try {
    b();
} catch(e) {
    alert('problem calling function b(): ' + e);
};

​function a(){
    alert('function a() called');
};

var b = function(){
    alert('function b() called');
};​

You will see (as in mentioned jsfiddle), that a() function is called properly even before the actual declaration, but b() is not available before the assignment.