# JavaScript Switch 语句 **switch 语句用于基于不同的条件来执行不同的动作。** ## JavaScript Switch 语句 请使用 switch 语句来选择要执行的多个代码块之一。 ### 语法 ``` switch(n) { case 1: 执行代码块 1 break; case 2: 执行代码块 2 break; default: n 与 case 1 和 case 2 不同时执行的代码 } ``` 工作原理：首先设置表达式 n（通常是一个变量）。随后表达式的值会与结构中的每个 case 的值做比较。如果存在匹配，则与该 case 关联的代码块会被执行。请使用 _break_ 来阻止代码自动地向下一个 case 运行。 ### 实例 显示今日的周名称。请注意 Sunday=0, Monday=1, Tuesday=2, 等等： ``` var day=new Date().getDay(); switch (day) { case 0: x="Today it's Sunday"; break; case 1: x="Today it's Monday"; break; case 2: x="Today it's Tuesday"; break; case 3: x="Today it's Wednesday"; break; case 4: x="Today it's Thursday"; break; case 5: x="Today it's Friday"; break; case 6: x="Today it's Saturday"; break; } ``` x 的结果： ``` <script>var d=new Date().getDay(); switch (d) { case 0: x="Today it's Sunday"; break; case 1: x="Today it's Monday"; break; case 2: x="Today it's Tuesday"; break; case 3: x="Today it's Wednesday"; break; case 4: x="Today it's Thursday"; break; case 5: x="Today it's Friday"; break; case 6: x="Today it's Saturday"; break; } document.write(x);</script> Today it's Tuesday ``` ## default 关键词 请使用 default 关键词来规定匹配不存在时做的事情： ### 实例 如果今天不是周六或周日，则会输出默认的消息： ``` var day=new Date().getDay(); switch (day) { case 6: x="Today it's Saturday"; break; case 0: x="Today it's Sunday"; break; `default: x="Looking forward to the Weekend";` } ``` x 的结果： ``` <script>var d=new Date().getDay(); switch (d) { case 6: x="Today it's Saturday"; break; case 0: x="Today it's Sunday"; break; default: x="Looking forward to the Weekend"; } document.write(x);</script> Looking forward to the Weekend ``` ## 课外书 如需更多有关 _JavaScript Switch 语句_的知识，请阅读 JavaScript 高级教程中的相关内容： [ECMAScript switch 语句](/js/pro_js_statements_switch.asp "ECMAScript switch 语句") switch 语句是 if 语句的兄弟语句。本节介绍了 switch 语句的用法，以及与 Java 中的 switch 语句的不同。