What is the difference between these two functions/approaches?

Question

I use only jQuery for writing JavaScript code. One thing that confuses me is these two approaches of writing functions,

First approach

vote = function (action,feedbackId,responseDiv)
{
    alert('hi');
    return feedbackId;
}

Second approach

function vote(action, feedbackId,responseDiv)
{
    alert('hi');
    return feedbackId;
}

What is the difference between the two and why should one use the first approach or the second approach?

Answer 1

The first is a function expression assigned to the vote variable, the second is a function declaration.

The main difference is that function statements are evaluated at parse time, they are available before its declaration at runtime.

See also:

• Named function expressions demystified (article)
• Explain JavaScript’s encapsulated anonymous function syntax

Answer 2

function myFunction() {}

...is called a "function declaration".

var myFunction = function() {};

...is called a "function expression".

They're very similar; however:

• The function declaration can be declared after it is referenced, whereas the function expression must be declared before it is referenced:

  // OK
  myFunction();
  function myFunction() {}
  
  // Error
  myFunction();
  var myFunction = function() {};
  

• Since a function expression is a statement, it should be followed by a semi-colon.

See Function constructor vs. function declaration vs. function expression at the Mozilla Developer Centre for more information.

Answer 3

The function declaration syntax cannot be used within a block statement.

Legal:

function a() {
    function b() {

    }
}

Illegal:

function a() {
    if (c) {
        function b() {

        }
    }
}

You can do this though:

function a() {
    var b;
    if (c) {
        b = function() {

        };
    }
}

Answer 4

The first one is a function expression,

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

alert(calculateSum(5, 5)); // Alerts 10

The second one is a plain function declaration.