Scope and variable hoisting in JavaScript

1. Variable domain

js only has function scope (local scope) and global scope (global scope), no block-level scope

2. Variable promotion

For variables defined by val and function a(){} , there is variable promotion.

Note: There is no variable promotion for function expressions.

Example 1:

(function() {

    

    console.log(testc); //print out undefined

testa(); // 打印出testa testb(); // 报错：提示undefined is not a function function testa() { console.log("testa"); } var testb = function() { console.log("tesb"); } var testc = "testc";})(); Example two:

var a = 1;
function b() {
     console.log(a);                                  //function a(){}
     a = 10;
     console.log(a); //10, the variable a is defined in the function scope.
    return;
    function a() { //The variable is promoted to the front, which is equivalent to var a=function(){}
    }
}
b();
console.log(a); // 1, access to global variables
 

Example three:

var a = 1;
function b() {
       console.log(a); //1, at this time a is a global variable
       a = 10;
       console.log(a); //10, changed the value of the global variable
       return;
       function c() {

       }
}
  b();
 console.log(a);                                          //10

Corrections are welcome!