Do you really understand the difference between adding var and not adding var in javascript_javascript skills

Javascript is a product that follows the ECMAScript standard, and naturally it must follow the ECMAScript standard.

Let’s first look at the definition and usage of the var keyword

The

var statement is used to declare variables.

The creation of JavaScript variables is also called "declaring" a variable:

Copy code The code is as follows:

var carName;

After a variable is declared, the variable is empty (has no value).

Copy variables, the operation is as follows:

Copy code The code is as follows:

carName = "Volvo";

When declaring a variable, you can also assign a value to the variable:

Copy code The code is as follows:

var carName = "Volvo";

Grammar

Copy code The code is as follows:

var varname = value;

Parameter value

Parameters	Description
varname	Must. Specify variable name. Variable names can contain letters, numbers, underscores and dollar signs. Variable names must start with a letter Variable names can also start with $ and _ (but generally not used this way) Variable names are case-sensitive (y and Y are different variables) Reserved words (such as JavaScript keywords) cannot be used as variable names
value	Optional. Specify the value of the variable. Note: If a variable declaration does not specify a value, its default value is undefined

Everyone has read many articles and they all say to avoid implicit declaration of global variables, which means that 'var' must be added before declaring a variable. So what is the difference between adding 'var' and not adding 'var'?

Let’s look at a piece of code first

var a = 'aa';
alert(a); //弹出 'aa'
alert(window.a)//弹出'aa' 

Understood, when you declare a global variable, you actually add an attribute to the 'window' object. The following piece of code has the same effect

a = 'aa';
alert(a); //弹出 'aa'
alert(window.a)//弹出'aa' 

Then "var a = 'aa' " and "a = 'aa' " are both global variables, what's the difference? Look at the following two pieces of code

var a = 'aa';
delete window.a; // false 
a = 'aa';
delete window.a; // true 

They all add attributes to the 'window' object, one can be deleted and the other cannot be deleted. But adding 'var' will make the scope related. Without 'var', you will always dynamically add attributes to the 'window' object. The following code proves it

var test = function(){
 a = 'aa';
}
test();
alert(window.a);//弹出'aa' 

Since the window object is a global object, it can be omitted by default. The following paragraph has the same effect

var test = function(){
 a = 'aa';
}
test();
alert(a);//弹出'aa' 

Speaking of this, students who are thinking seriously must have a question now, why can implicitly declared global variables be deleted, but explicitly declared global variables cannot be deleted?

The reason is that "delete cannot delete properties whose configurability is false". The properties of some built-in objects are not configurable, such as the properties of global objects created through variable declarations or function declarations. The following code proves it

delete Object.prototype; // false 不可删除，该属性是不可配置的
var a = 'aa';
delete window.a;//false 不可删除，该属性是不可配置的
function test(){};
delete window.test;//false 不可删除，该属性是不可配置的 

Then don’t you understand that the global variable declared with 'var' actually adds a non-configurable attribute to the 'window' object, while the global variable declared without 'var' actually adds a non-configurable attribute to the 'window' object. A configurable property is added to the 'window' object.

Note, wherever window is used above, window can be replaced by this, such as:

var test = function(){
 a = 'aa';
}
test();
alert(this.a);//弹出'aa' 

As for the reason, please check the article I wrote before 'This, this, discuss this in javascript again, super comprehensive'

The following pulls out the var keyword in javascript and explains it separately to everyone.

We know that when defining a variable, we need to use the Var keyword. When using the Var keyword, we need to pay attention to its usage:
The following examples fully illustrate that Var has different execution results when it is used or not used, and when global variables and local variables are defined.

var var01 = 1;
function funtest() {
 document.write(var01);
 var var01 = 0;
} 

The result is: undefined

var var01 = 1;
function funtest() {
 document.write(var01);
 var01 = 0;
} 

The result is: 1

 var01 = 1;
function funtest() {
 document.write(var01);
 var var01 = 0;
}

The result is: undefined

var01 = 1;
function funtest() {
 document.write(var01);
 var01 = 0;
}

The result is: 1

See how much you know about var in JavaScript here. I believe everyone will gain something from learning through this article. To learn more about javascript var, please continue to pay attention to this site, thank you!