js--scope of variables

As a rookie, it feels good to learn something every day. Today, when I was learning about closures, I saw the two concepts of scope and scope chain. I feel that as a novice with ambitions, it is necessary to understand them in detail.

The scope of variables

As far as js variables are concerned, there are global variables and local variables. I think this can be understood literally... Here's an example

var message = "今天我做的糯米蒸排骨"; //定义一个全局变量function doL(){

　　var ss = "侠客行很好看";      //定义一个局部变量

　　alert(message);             //输出"今天我做的糯米蒸排骨",在函数中可以引用到全局变量

　　function alertDo(){

　　　　alert(ss);

　　}

　　alertDo();             //输出"侠客行很好看",这就涉及变量的作用域了

};doL();
alertDo();              //没法输出  alertDo is not definedalert(message);         //可以输出"今天我做的糯米蒸排骨"

There are two very interesting places here

1 .When a variable is declared without var we are actually declaring a global variable. Well, this is wrong. See below, num is a global variable,

and mum = 1 ;

In fact, it is an attribute assignment operation. First, it will try to resolve the mum in the current scope chain (if declared in a method, the current scope chain represents the global scope and method local scope etc...); if it is in any current scope chain If mum is found in the scope chain, the assignment of the mum attribute will be performed; if mum is not found, it will be assigned to the global object (that is, the current scope chain Create the mum attribute in the top-level object (such as window object) and assign a value.

Notice! It does not declare a global variable, but creates a property of the global object. Since the variable declaration has the non-deletable attribute, compare var num = 1 with num = 1. The former is a variable declaration with the non-deletable attribute, so it cannot be deleted; the latter is an attribute of the global variable, so it can be deleted from the global variable. .

var num =1;
mum = 1;

2. The variable declaration will be advanced to the top of the function. In fact, we have encountered it before

    var scope="global";  function t(){  
        console.log(scope);  var scope="local"  
        console.log(scope);  
    }  
    t();

Undefined will be output first, then scope, which will declare variables in advance and overwrite local variables, which is equivalent to the following situation

    var scope="global";  function t(){  var scope；
        console.log(scope);  
        scope="local"  
        console.log(scope);  
    }  
    t();

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