Function Definition- Three Way to Define a JavaScript Function

There are 3 ways you can define a function.

1. Using Function constructor

var sum = new Function('a','b', 'return a + b;');
alert(sum(10, 20)); //alerts 30

2. Using Function declaration.

function sum(a, b)
{
    return a + b;
}

alert(sum(10, 10)); //Alerts 20;

3. Function Expression

var sum = function(a, b) { return a + b; }

alert(sum(5, 5)); // alerts 10

Difference between declaration and expression: 

From ECMA Script specification:

FunctionDeclaration : function Identifier ( FormalParameterListopt ){ FunctionBody }
FunctionExpression : function Identifieropt ( FormalParameterListopt ){ FunctionBody }

If you notice, 'identifier' is optional for function expression. And when you don't give an identifier, you create an anonymous function. It doesn't mean that you can't specify an identifier.
This means following is valid.

var sum = function mySum(a, b) { return a + b; }

Important point to note is that you can use 'mySum' only inside the mySum function body, not outside.

See following example:

var test1 = function test2() { alert(typeof test2); }

alert(typeof(test2)); //alerts 'undefined', surprise! 

test1(); //alerts 'function' because test2 is a function.