javascript document.getElementById（“ device”）不起作用？
[英]javascript document.getElementById(“device”) doesn't work?
问题描述
Why does alert(p1) show null ? 
为什么alert(p1)显示为null ？
http://pastebin.com/VAKwwEge http://pastebin.com/VAKwwEge
<!DOCTYPE html>
<html>
<head>
<script type="text/javascript" src="js/product.js"></script>
</head>
<body>
Device:<select id="device" name="device" style="width:250px;"> </select> Line:<select id="line" name="line" style="width:250px;"> </select>
</body>
</html>
window.onload = load();
function load() {
var p1 = document.getElementById("device");
var l1 = document.getElementById("line");
alert(p1);
alert(l1);
}
2 个解决方案
解决方案1
2 2011-12-27 07:02:45
Change your code to this: 将代码更改为此：
function load() {
var p1 = document.getElementById("device");
var l1 = document.getElementById("line");
alert(p1);
alert(l1);
}
window.onload = load;
The assignment to onload needs to be a function reference, not the execution of the function (no parens). 对onload的分配需要是一个函数引用，而不是函数的执行（无括号）。
Somewhat simpler would be using an anonymous function like this: 使用这样的匿名函数会更简单一些：
window.onload = function() {
var p1 = document.getElementById("device");
var l1 = document.getElementById("line");
alert(p1);
alert(l1);
}
解决方案2
2 已采纳 2011-12-27 07:03:14
You need to change this line: 您需要更改此行：
window.onload = load();
To read the following: 阅读以下内容：
window.onload = load;
Note the difference, the first example will invoke the load() function, and assign its return value to window.onload , which of course will be undefined . 注意区别，第一个示例将调用 load()函数，并将其返回值分配给window.onload ，该值当然是undefined 。
The second example will assign the function itself to window.onload , which is what you want, and will alert [object HTMLSelectElement] (in Firefox anyways). 第二个示例将函数本身分配给所需的window.onload ，并将警告[object HTMLSelectElement] （无论如何在Firefox中）。
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