Function Declaration VS Function Epression

Difference between Function Declaration and Function Expression:

What is a Function Declaration?

A Function Declaration defines a named function variable without requiring variable assignment. Function Declarations occur as standalone constructs and cannot be nested within non-function blocks. It’s helpful to think of them as siblings of Variable Declarations. Just as Variable Declarations must start with “var”, Function Declarations must begin with “function”.

e.g.

function bar() {

    return 3;

}

ECMA 5 (13.0) defines the syntax as 

function Identifier ( FormalParameterListopt ) { FunctionBody }

The function name is visible within it’s scope and the scope of it’s parent (which is good because otherwise it would be unreachable)

function bar() {

    return 3;

}

 

bar() //3

bar  //function

What is a Function Expression?

A Function Expression defines a function as a part of a larger expression syntax (typically a variable assignment ). Functions defined via Functions Expressions can be named or anonymous. Function Expressions must not start with “function” (hence the parentheses around the self invoking example below)

e.g.

//anonymous function expression

var a = function() {

    return 3;

}

 

//named function expression

var a = function bar() {

    return 3;

}

 

//self invoking function expression

(function sayHello() {

    alert("hello!");

})();

ECMA 5 (13.0) defines the syntax as

function Identifieropt ( FormalParameterListopt ) { FunctionBody }

(though this feels incomplete since it omits the requirement that the containing syntax be an expression and not start with “function”)

The function name (if any) is not visible outside of it’s scope (contrast with Function Declarations).