//Function declaration
function foo() { return 5; }

//Anonymous function expression
var foo = function() { return 5; }

//Named function expression
var foo = function foo() { return 5; }

function declarations loads before any code is executed.While function expressions loads only
when the interpreter reaches that line of code.

So if you try to call a function expression before it’s loaded, you’ll get an error

But if you call a function declaration, it’ll always work. Because no code can be called until all declarations are loaded.

Function Expression

  alert(foo()); // ERROR! foo wasn't loaded yet
  var foo = function() { return 5; } 

Function Declaration

  alert(foo()); // Alerts 5. Declarations are loaded before any code can run.
  function foo() { return 5; } 

The difference is in First case foo() is defined at run-time, whereas in second case foo() is defined at parse-time
for a script block