The difference between whether JavaScript uses var to define variables, with examples

Although javaScript has many similarities with java and c languages, there are also differences.

JavaScript variables do not have block scope. The only ones that have their own scope are functions.

(1) Look at the example below

<script type="text/javascript">
    //定义全局变量
    var scope="全集变量";    function test(){
        //全局变量被局部变量覆盖
        document.writeln(scope+"<br/>")        //定义scope局部变量，起作用范围是整个函数
        var scope="局部变量";        //再次输出scope值
        document.writeln(scope);
    }
    test();</script>

Run result:
undefined
Local variable

(2) Remove var

<script type="text/javascript">
    //定义全局变量
    var scope="全集变量";    function test(){
        //全局变量被局部变量覆盖
        document.writeln(scope+"<br/>")        //定义scope局部变量，起作用范围是整个函数
        scope="局部变量";        //再次输出scope值
        document.writeln(scope);
    }
    test();</script>

in test(). Running result:
Full set of variables
Local variables

(3 ) Reason analysis

The main reason is that there are differences between defining variables with var and without var:

①If you use var variables, then the program will forcefully define a new variable

②If If a variable is not defined using var, the system will first search whether the variable exists in the current context. Only if the variable does not exist, the system will redefine a new variable

The reason why the above changes from undefined to "Global variable" is because the code that defines the local variable does not define a new variable, but directly creates a global variable scope and assigns it a value. Therefore, test() does not cover the global variable scope, so it also outputs "global variables"

Although JavaScript has many similarities with Java and C languages, there are also differences.

JavaScript variables do not have block scope. The only ones that have their own scope are functions.

(1) Look at the example below

<script type="text/javascript">
    //定义全局变量
    var scope="全集变量";    function test(){
        //全局变量被局部变量覆盖
        document.writeln(scope+"<br/>")        //定义scope局部变量，起作用范围是整个函数
        var scope="局部变量";        //再次输出scope值
        document.writeln(scope);
    }
    test();</script>

Run result:
undefined
Local variable

(2) Remove var

<script type="text/javascript">
    //定义全局变量
    var scope="全集变量";    function test(){
        //全局变量被局部变量覆盖
        document.writeln(scope+"<br/>")        //定义scope局部变量，起作用范围是整个函数
        scope="局部变量";        //再次输出scope值
        document.writeln(scope);
    }
    test();</script>

in test(). Running result:
Full set of variables
Local variables

(3 ) Reason analysis

The main reason is that there are differences between defining variables with var and without var:

①If you use var variables, then the program will forcefully define a new variable

②If If a variable is not defined using var, the system will first search whether the variable exists in the current context. Only if the variable does not exist, the system will redefine a new variable

The reason why the above changes from undefined to "Global variable" is because the code that defines the local variable does not define a new variable, but directly creates a global variable scope and assigns it a value. Therefore, test() does not cover the global variable scope, so the "global variable" is output.

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