这是这些代码的效果，在框内输入数字选择加减乘除，进行运算。
| <!DOCTYPE html> | |
| <html> | |
| <head lang="en"> | |
| <meta charset="UTF-8"> | |
| <title></title> | |
| <style> | |
| </style> | |
| </head> | |
| <body> | |
| <div> | |
| javascript综合练习作业一 | |
| </div> | |
| <div> | |
| <input type="text" size="10" id="num1" /> | |
| <select name="sel" id="sel"> | |
| <option>+</option> | |
| <option>-</option> | |
| <option>*</option> | |
| <option>/</option> | |
| <option>%</option> | |
| </select> | |
| <input type="text" size="10" id="num2" /> | |
| <button onclick="myadd()">=</button> | |
| <input type="text" size="10" id="num3" readonly /> | |
| <div id="div11" style="color: red; font-size: 12px;" > | |
| </div> | |
| </div> | |
| <script> | |
| function myadd(){ | |
| var num1 = document.getElementById(‘num1‘); | |
| var num2 = document.getElementById(‘num2‘); | |
| var num3 = document.getElementById(‘num3‘); | |
| var errinfo = document.getElementById(‘div11‘); | |
| var sel = document.getElementById(‘sel‘); | |
| errinfo.innerHTML=‘‘; | |
| if(num1.value=="" || isNaN(num1.value) || num2.value=="" || isNaN(num2.value)){ | |
| errinfo.innerHTML=‘输入的内容不是数字‘; | |
| }else{ | |
| if(sel.value==‘+‘){ | |
| num3.value = (parseFloat(num1.value)+parseFloat(num2.value)).toFixed(2); | |
| }else if(sel.value==‘-‘){ | |
| num3.value = (parseFloat(num1.value)-parseFloat(num2.value)).toFixed(2); | |
| }else if(sel.value==‘*‘){ | |
| num3.value = (parseFloat(num1.value)*parseFloat(num2.value)).toFixed(2); | |
| }else if(sel.value==‘/‘){ | |
| if(parseFloat(num2.value)==0){ | |
| errinfo.innerHTML=‘ 被除数不能为0‘; | |
| }else{ | |
| num3.value = (parseFloat(num1.value)/parseFloat(num2.value)).toFixed(2); | |
| } | |
| }else if(sel.value==‘%‘){ | |
| if(parseFloat(num2.value)==0){ | |
| errinfo.innerHTML=‘ 被除数不能为0‘; | |
| }else{ | |
| num3.value = (parseFloat(num1.value)/parseFloat(num2.value)).toFixed(2); | |
| } | |
| } | |
| } | |
| } | |
| </script> | |
| </body> | |
| </html> |
原文：http://www.cnblogs.com/host717/p/6666145.html