What is the difference between a function expression vs declaration in JavaScript? [duplicate]

Answer 1

Function Declaration

function foo() { ... }

Because of function hoisting, the function declared this way can be called both after and before the definition.

Function Expression

1. Named Function Expression

   var foo = function bar() { ... }
   

2. Anonymous Function Expression

   var foo = function() { ... }
   

foo() can be called only after creation.

Immediately-Invoked Function Expression (IIFE)

(function() { ... }());

Conclusion

Crockford recommends to use function expression because it makes it clear that foo is a variable containing a function value. Well, personally, I prefer to use Declaration unless there is a reason for Expression.