On the promotion _javascript techniques of variable and function declarations in JavaScript

Phenomenon:

1. The Declaration of variables and functions in JavaScript is elevated to the top of execution.

2. The elevation of the function is higher than the variable.

3. If an external variable with the same name is declared with Var, the function will no longer look up.

4. Anonymous functions do not ascend.

5. The functions in different <script> blocks do not affect each other.

Example:

function declaration elevation above variable declaration

Declare both variable A and function a
var A;
function A () {} 
alert (typeof a);//displays "function", initially proving that the function has precedence over var.

//First declare the function after declaring the variable, prove that the above example is not a function to overwrite the variable
function a () {}
var A; 
Alert (typeof a); The display is still "function", not "undefined", that is, the function has precedence over var.

//Declaration of variables at the same time assignment
function A () {}
var a = 1;
Alert (typeof a); Number, it is not a function at this time.
//Description: "var a=1" is equivalent to "Var a;a=1", that is, the first declaration, after the assignment, "a=1" is equivalent to a new assignment, the nature is number!

The function internally uses Var to define the same variables as the outside, and the function will no longer look up the external variables

var value = ' Hello ';
Function Show () {
  alert (value);
  if (!value) {
    var value = ' function ';
  }
  alert (value);
}
Show ()//Here The call function pops "undefined" in turn, and the "function"

//example is equivalent to var value = ' Hello ';
Function Show () {
var value;//Note This line of
  alert (value);
  if (!value) {
    value = ' function ';//This line removes Var
  }
  alert (value);
}
Show ()//1. If the show inner definition value is not in Var, then an external variable will be used to pop "Hello" and "Hello". 
//2. If you do not define value inside a function, you can also get an external value value.

Anonymous functions do not ascend up

GetName ()
var getName = function () {
  alert (' Closule ')
}
function GetName () {
  alert (' function ')
}
getName ()

//Top of the code is equivalent to
functions GetName () {//The elevation of
  alert (' function ')
}
getName ()
var getName = function () {
  alert (' Closule ')
}
getName ()

//code execution pops up separately Function "," Closule "

Functions in different <script> blocks do not affect each other

<script>
  getName ()
  var getName = function () {
    alert (' Closule ')
  }
<script>
< script>
  function GetName () {
    alert (' function ')
  }
<script>

//Code Execution error: TypeError: GetName is not a function
//Because the GetName () function is undefined in the first <script> block, the anonymous function does not ascend up

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