设计HTML和JavaScript程序，要求在文本框中输入三个数字，按下查询按钮以后，利用alert输出三个数字之和

图片说明

index.html文件内容
<!DOCTYPE html>

    <input id="sum" type="button" value="测试" />
</body>

index.js文件内容
function sum() {
var button = document.getElementById("sum");
alert("abc");
button.onclick = function() {
var num1 = document.getElementById("num1");
var num2 = document.getElementById("num2");
var num3 = document.getElementById("num3");

    alert("三个数字之和为：" + (parseInt(num1.value) + parseInt(num2.value) + parseInt(num3.value)));
}

}

function addLoadEvent(func) {
var oldLoad = window.oldLoad;
if (typeof oldLoad != 'function') {
window.onload = func;
} else {
window.oldLoad = function() {
oldLoad();
func();
}
}
}

addLoadEvent(sum);


```<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8" />
        <title></title>
        <script type="text/javascript">
            var n1=document.getElementById("num1").value;   //获取到的是字符串,不能用n1进行运算
            var n2=document.getElementById("num2").value;
            var n3=document.getElementById("num3").value;
            console.log(typeof n1);     //输出的是unfinished类型
            function btn(){
                alert(parseFloat(document.getElementById("num1").value)+parseFloat(document.getElementById("num2").value)+parseFloat(document.getElementById("num3").value));
            }
        </script>
    </head>
    <body>
        <p>请输入三个数字：</p>
        <form action="index.html" method="get">
            <input type="text" id="num1" value="" />
            <input type="text" id="num2" value="" />
            <input type="text" id="num3" value="" />
            <button type="button" onclick="btn();">测试</button>
        </form>
    </body>
</html>