<form>

             第一个数字：<br>

             <input type="text" id="num1">

             <br>

             <br>

             第二个数字：<br>

             <input type="text" id="num2">

             <br>

             <button id="add">+</button>

             <button id="subtract">-</button>

             <button id="multiply">*</button>

             <button id="divide">/</button>

         </form>

 <script type="text/javascript">

             //获取加号按钮

             var addBtn = document.getElementById('add');

             var subtractBtn = document.getElementById('subtract');

             var multiplyBtn = document.getElementById('multiply');

             var divideBtn = document.getElementById('divide');

             //为按钮添加点击方法

             addBtn.onclick = function(){

                 var a = document.getElementById('num1').value;

                 var b = document.getElementById('num2').value;

                 var a_int = parseInt(a, 10);

                 var b_int = parseInt(b, 10);

                 //调用加法

                 var result = addition(a_int,b_int);

                 alert(result);

                 }

             subtractBtn.onclick = function(){

                 var a = document.getElementById('num1').value;

                 var b = document.getElementById('num2').value;

                 var a_int = parseInt(a, 10);

                 var b_int = parseInt(b, 10);

                 //调用减法

                 var result = substraction(a_int,b_int);

                 alert(result);

                 }

             multiplyBtn.onclick = function(){

                 var a = document.getElementById('num1').value;

                 var b = document.getElementById('num2').value;

                 var a_int = parseInt(a, 10);

                 var b_int = parseInt(b, 10);

                 //调用乘法

                 var result = multiplication(a_int,b_int);

                 alert(result);

                 }

             divideBtn.onclick = function(){

                 var a = document.getElementById('num1').value;

                 var b = document.getElementById('num2').value;

                 var a_int = parseInt(a, 10);

                 var b_int = parseInt(b, 10);

                 //调用减法

                 var result = division(a_int,b_int);

                 alert(result);

                 }

             //构造函数

             function addition(x, y){

                 return x + y;

             }

             function substraction(x, y){

                 return x - y;

             }

             function multiplication(x, y){

                 return x * y;

             }

             function division(x, y){

                 if(y==0){

                     alert('0不能做除数');

                     return;

                 } else {

                     return x / y;

                 }

             }

         </script>

<script type="text/javascript">

            //获取加号按钮

            var addBtn = document.getElementById('add');

            var subtractBtn = document.getElementById('subtract');

            var multiplyBtn = document.getElementById('multiply');

            var divideBtn = document.getElementById('divide');

            //定义两个全局变量

            var a_int;

            var b_int;

            //为按钮添加点击方法

            addBtn.onclick = function(){

                getInputNum();

                //调用加法

                var result = addition(a_int,b_int);

                alert(result);

                }

            subtractBtn.onclick = function(){

                getInputNum();

                //调用减法

                var result = substraction(a_int,b_int);

                alert(result);

                }

            multiplyBtn.onclick = function(){

                getInputNum();

                //调用乘法

                var result = multiplication(a_int,b_int);

                alert(result);

                }

            divideBtn.onclick = function(){

                getInputNum();

                //调用减法

                var result = division(a_int,b_int);

                alert(result);

                }

            function getInputNum(){

                var a = document.getElementById('num1').value;

                var b = document.getElementById('num2').value;

                //将转化后的数字赋值给全局变量

                a_int = parseInt(a, 10);

                b_int = parseInt(b, 10);

            }

            //构造函数，加减乘除

            function addition(x, y){

                return x + y;

            }

            function substraction(x, y){

                return x - y;

            }

            function multiplication(x, y){

                return x * y;

            }

            function division(x, y){

                if(y==0){

                    alert('0不能做除数');

                    return;

                } else {

                    return x / y;

                }

            }

        </script>