getElementById() returns null even though the element exists [duplicate]

This question already has an answer here:

Why does jQuery or a DOM method such as getElementById not find the element? 8 answers

I'm trying to get the element with getElementById(), but it returns null even though the element exists. What am I doing wrong?

<html>
<head> 
    <title>blah</title>
    <script type="text/javascript">
        alert(document.getElementById("abc"));
    </script>
</head> 
<body>
    <div id="abc">

    </div>
</body>

You have to put this in a document load event. The DOM hasn't gotten to abc by the time the script is executed.

Your script runs before the DOM has been loaded. To fix this you can place your code in the window.onload function like so:

window.onload = function() {
    alert(document.getElementById("abc"));
};

An alternative is to place your script right before the closing </body> tag.

If you don't want to attach to the load event then simply put your script at the bottom of the body, so it will execute at the end-

<html>
<head> 
    <title>blah</title>    
</head> 
<body>
    <div id="abc">
    </div>
    <script type="text/javascript">
        alert(document.getElementById("abc"));
    </script>
</body>
</html>

This is because the script runs before the page has rendered.

For proof add this attribute to the body tag:

<body onload="alert(document.getElementById('abc'));" >

But it doesn't exist, not at that point in the HTML. HTML documents are parsed top-to-bottom, just like programs run. The best solution is just to put your script tag at the bottom of the page. You could also attach your JavaScript to the onload event.

来源：https://stackoverflow.com/questions/5371047/getelementbyid-returns-null-even-though-the-element-exists

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