The difference between: var functionName = function() {} and function functionName() {} in Javascript?

There are following two to declare function in JavaScript 

function declaration:

function foo(){} // foo();

Function declaration  is a normal function you can call it normally like other function calling by its name.
 

function expression:

var someVariable = function() {  //do your code };

Function expression is also known as  “anonymous function” because  it does not any name. When you need to call function expression you need to declare it to variable. then you can access function using that variable.