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JavaScript variable scope analysis (detailed explanation)

青灯夜游

Release： 2018-09-13 17:50:42

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This chapter brings you JavaScript variable scope analysis (detailed explanation), so that you can learn some little knowledge about JavaScript scope. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

What is JavaScript scope?

In JavaScript, scope is a collection of accessible variables, objects, and functions.

The scope can be modified within the function.

JavaScript local scope

Variables are declared within the function and are local variables (local scope)

local Variables: can only be accessed inside the function.

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <button onclick="myFunction(1,2)">试一试</button>
    <p id="demo"></p>
</body>
<script type="text/javascript">
    function name() {
        //在此处声明一个变量
        var a = 10;
        //函数内可以调用 a
    }
    //此处（函数外）不能调用变量 a　　
    //因为局部变量只作用于函数内，所以不同的函数可以使用相同名称的变量名　　
    //局部变量在函数开始执行时创建，函数执行完毕后，变量会自动销毁
</script>
</html>

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JavaScript global variables

Variables defined outside the function are global variables.

Global variables have global scope and can be used by all scripts and functions in the web page.

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <button onclick="myFunction(1,2)">试一试</button>
    <p id="demo"></p>
</body>
<script type="text/javascript">
    /* 在此处声明一个全局变量 */
    var a = 10;
    function name() {
        //函数内可以调用 a
    }
    //此处也能调用变量 a
</script>
</html>

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If the variable is not declared within the function (not declared using the var keyword), the change amount is a global variable.

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <button onclick="myFunction(1,2)">试一试</button>
    <p id="demo"></p>
</body>
<script type="text/javascript">
    
    function name() {
        /* 在此处声明一个变量 */
          a = 10;
　　　　　/* a没有使用var关键字声明，则默认为全局变量 */

    }
    
</script>
</html>

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JavaScript variable life cycle

The life cycle of a variable is initialized at the time of its declaration. Local variables are destroyed after the function completes execution. Global variables are destroyed after the page is closed.

Function parameters

Function parameters only work within the function and are local variables.

Global variables in HTML

In HTML, global variables are window objects, and all data variables belong to the window object.

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <button onclick="myFunction(1,2)">试一试</button>
    <p id="demo"></p>
</body>
<script type="text/javascript">
    function name() {
         a = 10;
    }
    //此处可使用window.a调用变量 a
</script>
</html>

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Global variables or functions can override the variables or functions of the window object. Local variables include window objects and can override global variables and functions.

The let keyword and the const keyword are provided in es6

The declaration method of let is the same as that of var. Using let instead of var to declare variables can limit the current variable to the code block. .

Using const to declare a constant, its value cannot be changed once it is set.

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