About variables in JavaScript

1. The variable declaration statement of JS is always executed first in its scope, regardless of the location where it is defined;
2. Inside the function body, local variables have a higher precedence than global variables with the same name.
3. Javas Scope classification
4. Variables that are not defined with Var, regardless of where they are defined, are global variables, and the global variables are essentially properties under window

A variable declaration statement of 1 JS is always executed first in its scope, regardless of where it is defined

2 var function ()3  {4  console.log (s);//output is undefined5var s= "You" ; 6 }; 7 test (); 　　　　　 　　　　　　                

The reason is that in the scope of the test function, although Var s= "You" behind the console, but because the declaration statement of the variable will be executed first, it is equivalent to:

 2  var  test= function    3   { 4  var  s;//  5  Conso Le.log (s); //  output is undefined  6  var  s= "You" ; 7  };                    8  test (); 

Console.log (s) results for undefined;

2 inside the function body, the precedence of the local variable is higher than the global variable with the same name

        var s= "Hello";　　　　　　 varfunction() {  var s= "You"; Console.log (s); // the output is "you"　　　　 }; Test ();

The local variable takes precedence, so the print result is "you"

Scope Classification of 3JS

There are only two types of JavaScript scope classifications: Global scope and Method scope

Functions are also called methods, so the scope of a method can be understood as a function within a scope, unlike C or C + + language, JS does not have block-level scope, for (Var i=0; i++) The I scope here is the scope of the function where the for loop is located, not just within the for loop;

4 variables that are not defined with Var, regardless of where they are defined, are global variables

 function Test ()// define functions test{  i=100;          } Test ()// Execute function testconsole.log (i);//Output 100
Console.log (WINDOW.I);//Output 100

In addition, it is necessary to note that function xxx () {}; Only the definition of a XXX (), and no function code, if there is no test (), then the output of I is undefined, can also be

function xxx () {//code};xxx (); merge written as: (function  XXX () { // Code}) ():

The form: (function xxx () {}) () is the function xxx is defined first and executes the function; The advantage of this method is that it can guarantee the variable lifetime, less global variables, and easy to recycle the variables.

About variables in JavaScript