JavaScript scope

In JavaScript, objects and functions are also variables.

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

JavaScript function scope: The scope is modified within the function.

#In JavaScript, variables declared with var actually have scope.

If a variable is declared inside the function body, the scope of the variable is the entire function body, and the variable cannot be referenced outside the function body:

'use strict';function foo () {
var x = 1;
x = x + 1;
}

x = x + 2; // ReferenceError! Unable to reference variable x# outside the function body

##If two different functions declare the same variable, the variable will only work within the respective function body. In other words, variables with the same name inside different functions are independent of each other and do not affect each other:

'use strict';function foo() { var x = 1;
x = x + 1;
}function bar() {
var x = 'A';
x = x + 'B';
}

##JavaScript Local scope

Variables are declared within the function, and the variables are local scope.

Local variables: can only be accessed inside the function.

// The carName variable cannot be called here

function myFunction() {
var carName = "Volvo";
// The carName variable can be called within the function
}

Note:

Because local variables only act within the function, different functions can use variables with the same name.

Local variables are created when the function starts executing, and will be automatically destroyed after the function is executed.

Global scope

Variables not defined within any function have global scope. In fact, JavaScript has a global object window by default, and variables in the global scope are actually bound to a property of window:

'use strict';

var course = 'Learn JavaScript';

##alert(course); // 'Learn JavaScript'

alert(window. course); // 'Learn JavaScript'

Therefore, directly accessing the global variable course is exactly the same as accessing window.course.

<!DOCTYPE html>
<html>
<head> 
<meta charset="utf-8"> 
<title>php中文网(php.cn)</title> 
</head>
<body>
<p>全局变量在任何脚本和函数内均可访问。</p>
<p id="demo"></p>
<script>
var carName = "Volvo";
myFunction();
function myFunction() 
{
    document.getElementById("demo").innerHTML =
"这里是 " + carName;
}
</script>
</body>
</html>

JavaScript variable life cycle

JavaScript variable life cycle is initialized when it is declared.

Local variables are destroyed after the function is executed.

Global variables are destroyed after the page is closed.

Function parameters

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

Continuing Learning

new file

<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>php中文网(php.cn)</title> <script type="text/javascript"> var a =1; function test(){ alert(a); //a为undefined! 这个a并不是全局变量，这是因为在function scope里已经声明了（函数体倒数第4行）一个重名的局部变量, //所以全局变量a被覆盖了，这说明了Javascript在执行前会对整个脚本文件的定义部分做完整分析,所以在函数test()执行前, //函数体中的变量a就被指向内部的局部变量.而不是指向外部的全局变量. 但这时a只有声明，还没赋值，所以输出undefined。 a=4 alert(a); //a为4,没悬念了吧？这里的a还是局部变量哦！ var a; //局部变量a在这行声明 alert(a); //a还是为4,这是因为之前已把4赋给a了 } test(); alert(a); //a为1，这里并不在function scope内，a的值为全局变量的值 </script> </head> <body> </body> </html>

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JavaScript scope

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