JavaScript支持以下 if..else 语句形式-

  • if

  • if ... else 

  • if...else if...

if 语句

if 语句是基本的控制语句,它使JavaScript可以有条件地进行决策和执行语句。

基本if语句的语法如下:

if (expression) {
   Statement(s) to be executed if expression is true
}

在这里判断一个JavaScript表达式。如果输出值为true,则执行给定的语句。如果表达式为false,则不执行任何语句。

请尝试以下示例,以了解 if 语句的工作方式。

<html>
   <body>     
      <script type = "text/javascript">
         <!--
            var age = 20;
         
            if( age > 18 ) {
               document.write("<b>Qualifies for driving</b>");
            }
         //-->
      </script>      
      <p>Set the variable to different value and then try...</p>
   </body>
</html>

运行上面代码输出

Qualifies for driving
Set the variable to different value and then try...

if...else 语句

if ... else 语句是控制语句的另一种形式,它允许JavaScript以更受控制的方式执行语句。

if (expression) {
   Statement(s) to be executed if expression is true
} else {
   Statement(s) to be executed if expression is false
}

在此判断JavaScript表达式。如果输出值为true,则执行'if'块中的给定语句。如果表达式为假,则执行else块中的给定语句。

尝试以下代码,以了解如何在JavaScript中实现if-else语句。

<html>
   <body>   
      <script type = "text/javascript">
         <!--
            var age = 15;
         
            if( age > 18 ) {
               document.write("<b>Qualifies for driving</b>");
            } else {
               document.write("<b>Does not qualify for driving</b>");
            }
         //-->
      </script>     
      <p>Set the variable to different value and then try...</p>
   </body>
</html>

运行上面代码输出

Does not qualify for driving
Set the variable to different value and then try...

if...else if... else 语句

if ... else if ... 语句是 if ... else 的高级形式,可让JavaScript根据多种条件做出正确的决定。

if (expression 1) {
   Statement(s) to be executed if expression 1 is true
} else if (expression 2) {
   Statement(s) to be executed if expression 2 is true
} else if (expression 3) {
   Statement(s) to be executed if expression 3 is true
} else {
   Statement(s) to be executed if no expression is true
}

尝试以下代码,以了解如何在JavaScript中实现if-else if 示例。

<html>
   <body>   
      <script type = "text/javascript">
         <!--
            var book = "maths";
            if( book == "history" ) {
               document.write("<b>History Book</b>");
            } else if( book == "maths" ) {
               document.write("<b>Maths Book</b>");
            } else if( book == "economics" ) {
               document.write("<b>Economics Book</b>");
            } else {
               document.write("<b>Unknown Book</b>");
            }
         //-->
      </script>      
      <p>Set the variable to different value and then try...</p>
   </body>
<html>

运行上面代码输出

Maths Book
Set the variable to different value and then try...

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