作业如下
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>简易计算器</title><style>input {height: 30px; line-height: 30px; padding: 0 10px; width: 60px; border: solid 1px #ddd}select {height: 30px; line-height: 30px; padding: 0 10px;border: solid 1px #ddd}button {background-color: dodgerblue; color: #fff; border: none; height: 30px; line-height: 30px; padding: 0 10px; cursor: pointer}#jieguo {margin-top: 10px; color: #f00}</style></head><body><input type="text" name="a" id="a" value=""><select name="y" id="y"><option value="+">+</option><option value="-">-</option><option value="*">*</option><option value="/">/</option></select><input type="text" name="b" id="b" value="2"><button id="btn">计算</button><div id="jieguo">?</div><script>document.getElementById('btn').onclick=function () {var a = document.getElementById('a').value;var b = document.getElementById('b').value;var y = document.getElementById('y').value;a = parseInt(a);b = parseInt(b);var tiaojian = true; // 输入条件符合var n;// 输入情况判断if (!a){n = '第一个值不能为空或者不是数字';tiaojian = false}if (!b){var n = 1111;alert('第二个值不能为空或者不是数字')tiaojian = false}// if判断法if (tiaojian==true){if (y=='+'){n = a+"+"+b+"="+(a+b);}else if (y=='-'){n = a+"-"+b+"="+(a-b);}else if (y=='*'){n = a+"*"+b+"="+(a*b);}else if (y=='/'){n = a+"/"+b+"="+(a/b);}}document.getElementById('jieguo').innerHTML=n;}</script></body></html>
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