JavaScript基础语法（变量）

比眉伴天荒 2024-03-31 12:53 75阅读 0赞

## JavaScript基础语法（变量） ##

**学习路线**：[JavaScript基础语法（输出语句）][JavaScript]\->[JavaScript基础语法（变量）][JavaScript 1]\->[JavaScript基础语法（数据类型）][JavaScript 2]\->[JavaScript基础语法（运算符）][JavaScript 3]\->[JavaScript基础语法（流程控制语句）][JavaScript 4]\->[JavaScript基础语法（类型转换）][JavaScript 5]\->[JavaScript基础语法（函数）][JavaScript 6]\->[Array\_JavaScript][Array_JavaScript]\->[String\_JavaScript][String_JavaScript]\->[自定义对象\_JavaScript][JavaScript 7]\->[JavaScript\_BOM][JavaScript_BOM]\->[Window对象][Window]\->[confirm()、setInterval()、setTimeout()][confirm_setInterval_setTimeout]\->[History、Location][History_Location]\->[闪烁的灯泡][Link 1]\->[JavaScript\_DOM][JavaScript_DOM]\->[事件绑定(onclick,onfocus,onblur)][onclick_onfocus_onblur]\->  
[事件绑定(onmouseout,onmouseover)  
][onmouseout_onmouseover_]\->[事件绑定(onsubmit)表单提交  
][onsubmit_]\->[提交表单与验证表单案例  
][Link 2]

### 变量命名规范 ###

和`java`语言基本都相同

*  组成字符可以是任何字母、数字、下划线（\_）或美元符号（$）
 *  数字不能开头
 *  建议使用驼峰命名

### var ###

*  **作用域**：全局变量

<!DOCTYPE html>
    <html lang="en">
    <head>
      <meta charset="UTF-8">
      <title>Title</title>
    </head>
    <body>
      
    <script>
      // 在一个单独的作用域里面定义
      {
        
          var a = 1
      }
      console.log(a)
    </script>
    </body>
    </html>

**运行结果**  
![在这里插入图片描述][b7d44cc63c914f9a9cc219299ea391a9.png]

*  变量可以重复定义

<!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>Title</title>
    </head>
    <body>
        
    <script>
        // 在一个单独的作用域里面定义
        {
        
            var a = 1
            var a = 2
        }
        console.log(a)
    </script>
    </body>
    </html>

**运行结果**  
![在这里插入图片描述][05f22b7a454d416599ff7fe11dd31a31.png]

### let ###

这是`ES6`新增加的语法。

*  **作用域**：局部变量

<!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>Title</title>
    </head>
    <body>
        
    <script>
        // 在一个单独的作用域里面定义
        {
        
            let a = 1;
        }
        alert(a); 
    </script>
    </body>
    </html>

**运行结果**  
![在这里插入图片描述][9b17734d2ba34c0da603b5ff608d9968.png]

*  变量不可以重复定义

![在这里插入图片描述][09fffd60a6f94c4c9fac7d5d0980887e.png]

[JavaScript]: https://blog.csdn.net/qq_51447496/article/details/128085137
[JavaScript 1]: https://blog.csdn.net/qq_51447496/article/details/128085348
[JavaScript 2]: https://blog.csdn.net/qq_51447496/article/details/128086148
[JavaScript 3]: https://blog.csdn.net/qq_51447496/article/details/128086681
[JavaScript 4]: https://blog.csdn.net/qq_51447496/article/details/128088817
[JavaScript 5]: https://blog.csdn.net/qq_51447496/article/details/128087248
[JavaScript 6]: https://blog.csdn.net/qq_51447496/article/details/128088986
[Array_JavaScript]: https://blog.csdn.net/qq_51447496/article/details/128089474
[String_JavaScript]: https://blog.csdn.net/qq_51447496/article/details/128089631
[JavaScript 7]: https://blog.csdn.net/qq_51447496/article/details/128089766
[JavaScript_BOM]: https://editor.csdn.net/md/?articleId=128095193
[Window]: https://blog.csdn.net/qq_51447496/article/details/128095300
[confirm_setInterval_setTimeout]: https://blog.csdn.net/qq_51447496/article/details/128103458
[History_Location]: https://blog.csdn.net/qq_51447496/article/details/128104240
[Link 1]: https://blog.csdn.net/qq_51447496/article/details/128095524
[JavaScript_DOM]: https://blog.csdn.net/qq_51447496/article/details/128104720
[onclick_onfocus_onblur]: https://blog.csdn.net/qq_51447496/article/details/128110532
[onmouseout_onmouseover_]: https://blog.csdn.net/qq_51447496/article/details/128112439
[onsubmit_]: https://blog.csdn.net/qq_51447496/article/details/128112553
[Link 2]: https://blog.csdn.net/qq_51447496/article/details/128112770
[b7d44cc63c914f9a9cc219299ea391a9.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/8adb572d497e4ec48b78248a195088bc.png
[05f22b7a454d416599ff7fe11dd31a31.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/c5f213e2cf33447f87bd36e35f0d9126.png
[9b17734d2ba34c0da603b5ff608d9968.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/7524f44889e9403abdaad821b7a44601.png
[09fffd60a6f94c4c9fac7d5d0980887e.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/c58e43e9b7564e0c854c20183b53595d.png