function declarations and function expressions

The two look very much alike.

Functions declaration: Function Function name () {}

Functional Expressions: Function name (writable and not writable) () {}: Named function expression/anonymous function expression

For a few examples:

function aaa () {}: Functions declaration

var a=function aaa () {}: named function expression

var a = function () {}: an anonymous function expression;

(function aaa () {}): expression

Difference:

1. Function expressions can be directly followed by parentheses, and function declarations do not

function aaa () {alert (1);} () (Error)

var a=function aaa () {alert (1);} () (Run directly)

2. function declaration can be resolved in advance

if (true) {function aaa () {alert (1);}}

else{function aaa () {alert (2);}};

AAA (); The execution result is indeterminate, possibly 1, possibly 2, because the function declaration is pre-parsed and AAA is parsed before the IF

if (true) {var a=function aaa () {alert (1);}}

else{var a= function aaa () {alert (2);}};

A (); The execution result is 1 first executed if statement.

3. The difference between anonymous and named functions

var a=function aaa () {alert (1);}

A (); All browsers support

AAA (), non IE error but put AAA in the function can be. IE will parse it into a () and AAA ();

(function aaa () {alert (1)})

AAA () Non IE error.

function declarations and function expressions