JavaScript支持以下 if..else 语句形式-
if
if ... else
if...else if...
链接:https://www.learnfk.comhttps://www.learnfk.com/javascript/javascript-ifelse.html
来源:LearnFk无涯教程网
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 ... 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...
祝学习愉快!(内容编辑有误?请选中要编辑内容 -> 右键 -> 修改 -> 提交!)
HarmonyOS快速入门与实战 -〔QCon+案例研习社〕