Question

1. $(function () {
       function foo() { return true; }
       log(bar());                        // getting error
       var bar = function() { return true; };   
  });

2. $(function () {
       function foo() { return true; }
       var bar = function() { return true; };
       log(bar());                        // Working   
  });

my confusion here is what is the difference between the below two declarations and which one is useful?

var bar = function() { return true; };

function bar(){ return true; };

Answer 1

The:

function bar(){ return true; };

is function declaration which is hoisted to top by the interpreter, you can call from any place, while:

var bar = function() { return true; };

is function expression you can call only after it is defined. It won’t be available before or up in the code just like you were doing:

log(bar());                        // getting error
var bar = function() { return true; }; 

Getting error because on first line bar isn’t available yet. To solve that, use function declaration instead if you want.

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To learn more about the difference between function declaration and function expression, I highly recommend you to read this great article by Kangax:

• Named function expressions demystified