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This article mainly explains "what is the role of var in JavaScript". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Now let the editor take you to learn "what is the role of var in JavaScript"?

The essence of var

The essence of var is to declare variables.

Var str = 'China';// declares and assigns the scope of a variable

First, an example is given to illustrate the problem. In this example, the var keyword is not used to declare the variable in the function t, and the system thinks that it is only an assignment operation, and this assignment operation will affect the str2 property of the window object.

Var str1 = 'China';// declare and assign str1var str2 =' Japan';// declare and assign str2console.log (window.str1,window.str2); / / output: China Japanfunction t () {var str1 = 'local';// declare and assign str1 str2 =' global';// assignment, not declare variables, pollute the attributes of the window object} t (); / / call the t function console.log (window.str1,window.str2) / / output: the scope of the China global variable

Global scope: variables defined in the outermost layer

Local scope: variables defined within a function

The characteristics of JS scope: first look for variables inside the function, then find the outer function if you can't find it, gradually find the outermost layer, that is, window object, and manipulate the properties of window object.

Var str = 'China';function T1 () {var str =' Japan'; function T2 () {var str = 'USA'; console.log (str);} T2 ();} T1 ()

This example is typical, which can be used to illustrate the scope switching of JS variables. the script first defines the str attribute in the outermost layer. When the T1 function is called, the str variable and T2 method are also defined in the T1 function. When the T2 method is executed, the str variable is defined, so USA is printed at this time.

When the defined operation in the T2 function is masked and there is no str variable in the T2 function scope, it is found in its upper scope, and because the str variable is defined in the T1 function, the Japan is printed.

When masking the definition operations in the T1 and T2 functions, look out layer by layer, find the outermost layer, and print China at this time.

When the outermost layer of the variable is not defined, the variable cannot be found while running the JS, and an error can be found in the browser's console.

At this point, I believe you have a deeper understanding of "what is the role of var in JavaScript". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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