JS function-A grudge between a function declaration and a function expression

"Function declarations" and "function expressions" are actually the way to define functions

function declaration:

Function name () {...}

a function defined by the above method is a function declaration

function Expression: Functions are defined by function expressions in more ways

1. Var a = function test () {...}  //  This is a named function expression

 var A = function () {...}//This is an anonymous functional expression 

2.   functions defined by function declaration   enclosed in a pair of parentheses, which also forms the function expression

 (function test () {...}) This is also the function expression 

3.  and "function declaration" The pre-bitwise operator   can also form a function expression

For example: These are function expressions

~function Test () {...} +function Test () {...} -function Test () {...}! function Test () {...}

difference: What is the difference between a function declaration and a function expression??

1. Function expressions can be executed directly after the parentheses (this is the function self-executing), and the function declaration can not

For example:

var a = function test () {        alert ("Hello");}//This is a function expression that is appended to the expression to automatically execute the function.

var a = function test () {        alert ("Hello");//Refresh page pops up Hello} ();

And the way the function is declared is not allowed to execute, for example, the browser will prompt that the notation is incorrect.

function test () {        alert ("Hello");} ();

2. Function declarations can be pre-parsed, and function expressions do not; If you do not understand JS pre-analytic small partners, please refer to another article "JS Pre-analysis"

For example:

Window.onload = function () {        test ();        function test () {//Functions Declaration            alert ("Hello");}        }

A function that is defined by a function declaration can be pre-parsed, so calling the test () function before function test () naturally pops up Hello,

Window.onload = function () {        a ();        var a = function () {//Functions Expression            alert ("Hello");}        }

A function defined by a function expression cannot be pre-parsed, so calling a () before the function will result in an error.

let's look at an example:  

Window.onload = function () {        a ();       if (true) {           function A () {               alert ("1");           }       } else{           function A () {              alert ("2");           }       }}

We were going to let ifset up when the popup1, does not set up popup2

But because JS will be pre-parsed, so it always pops up 2, but using a function expression can avoid this situation

Window.onload = function () {       if (true) {           var a = function () {               alert ("1");           }       } else{           var a = function () {              alert ("2");           }       }       A (); }

since the function expression will not be pre-parsed, it will follow normal logic, and naturally it will pop up 1

so remember : When writing a program, if you need to decide to perform different functions according to different criteria, then you must define the function in the form of "function expression" . This will prevent many errors from occurring.

JS function-A grudge between a function declaration and a function expression