Three ways to define functions in JavaScript & scope of variables

I. Function definition

Mode 1. Normal way definition function

Function name (number of parameters N) {

function body

}

function Add (A, b) {return a+b;}

Method 2. Direct Volume definition function

var function name =function (number of parameters N) {

function body

}

var add=function (A, b) {return a+b;}

Mode three. constructor definition function

var function name =new function (parameter n, function body);

var add = new Function (' A, B ', ' return a+b; ');

Two. Scope of variables:

Global variables: Global domain and function domain

Local variables: within local function domains

<script type= "Text/javascript" >var a= "a"; function fn () {var b= "B"; alert (a); alert (b);} FN (); alert (a); ALELRT (b);</script>

Printed results: A,b,a We found the last alert (b) was not printed this is because the B definition in the function body can only be used within the function body
Special scopes

<script type= "Text/javascript" >var a= "a"; function fn () {alert (a); var a= "B"; alert (a);} Alert (a);</script>

Execution Result:

Undefined,b,a

Code Explanation:

* Read the code:
*  * defines a global variable A and initializes it to "a".
*  * defines a function, FN (), but it is not called (the code that describes the contents of the function does not run).
*    * define local variable A, but not initialize.
*  * to the above code, there are several variables in the current environment?

*    * global variable A, with a value of "a".
*    * local variable A, but no value.
*  * when the function fn () is called:
*    * Print Variable A: Nearest principle
*      * local variable A, no value: The result is undefined.
*    * print variable A, local variable a.
*  * print variable A: global variable A in the global domain.
* 
* Conclusion:
*  * JavaScript defines all the variables in a code snippet at once, but does not initialize it.
*  * JavaScript initializes the variable when it is run to the appropriate statement.
*  
*  * When the global variable has the same name as a local variable:
*    * in the function field, only the local variable A is available.

Three ways to define functions in JavaScript & scope of variables