This question already has an answer here:
<html>
<head>
<title>Test javascript</title>
<script type="text/javascript">
var e = document.getElementById("db_info");
e.innerHTML='Found you';
</script>
</head>
<body>
<div id="content">
<div id="tables">
</div>
<div id="db_info">
</div>
</div>
</body>
</html>
If I use alert(e); it turns up null.... and obviously I don't get any "found you" on screen. What am I doing wrong?
The problem is that you are trying to access the element before it exists. You need to wait for the page to be fully loaded. A possible approach is to use the onload handler:
window.onload = function () {
var e = document.getElementById("db_info");
e.innerHTML='Found you';
};
Most common JavaScript libraries provide a DOM-ready event, though. This is better, since window.onload waits for all images, too. You do not need that in most cases.
Another approach is to place the script tag right before your closing </body>-tag, since everything in front of it is loaded at the time of execution, then.
Run the code either in onload event, either just before you close body tag.
You try to find an element wich is not there at the moment you do it.
How will the browser know when to run the code inside script tag? So, to make the code run after the window is loaded completely,
window.onload = doStuff;
function doStuff() {
var e = document.getElementById("db_info");
e.innerHTML='Found you';
}
The other alternative is to keep your <script...</script> just before the closing </body> tag.
Script is called before element exists.
You should try one of the following:
- wrap code into a function and use a body onload event to call it.
- put script at the end of document
- use defer attribute into script tag declaration
The script is performed before the DOM of the body is built. Put it all into a function and call it from the onload of the body-element.
来源：https://stackoverflow.com/questions/4498482/javascript-cant-find-element-by-id