function declarations and function expressions

1. What is a function declaration, function expression

Functions declaration: Function Function name () {}

Functional expression: Function name () {}. The function name can be written without writing. Written, is named function expression, do not write is anonymous function expression

Example:

function aaa () {};

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

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

The following are all function expressions

(function aaa () {})

~function aaa () {}
-function aaa () {}
+function aaa () {}
!function aaa () {}

2. Differences

(1) The function expression can be executed directly after the parentheses, and the function declaration is not possible.

function aaa () {alert (1);} ();//function declaration not possible, result has error var a=function aaa () {alert (1);} ();//function expression Yes, result pop-up 1

~ function aaa () {alert (1);} ();//function expression Yes, result also popup 1

(2) function declaration can be resolved in advance

As in the following example: 1 is popped in Firefox, but 2 is popped in IE, but there is no such thing as a function expression. So in this case, use the function expression.

if (true) {    function aaa () {        alert (1);    }} else{        function aaa () {        alert (2);    }} AAA ();

Using function Expressions:

if (true) {    var a = function aaa (){        alert (1);    }} else{        var a = function aaa (){        alert (2);    }} a ();//with AAA (); Can you? No!

All that pops up is 1.

So the question is, why not use AAA (), because the AAA () is not found outside the call, but can be found in the interior.

For function expressions, do not use;

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

Results show

var a = function aaa () {    alert (1);    Alert (typeof AAA);  Inside is the}a () that can be found;/*aaa ();  I can't find it outside.

function declarations and function expressions