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Asked: 2026-06-05T18:03:54+00:00

The order of execution of html/javascript , as far as I know , is

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The order of execution of html/javascript , as far as I know , is serial (as in all programming language) , meaning the browser reads line by line of html/javascript code and interprets it .

That is why some javascript programmers put the tag at the end of the body – to make sure the entire DOM was loaded and they can now access whatever elements they want .
This is how they do it – 

<html>
...
<body>
...
<script> //whatever code you want to put
</script>
</body>

I wanted to test this so I wrote the following code

<html>
<head>
</head>
<body>
<script>
var elm=document.getElementById("myp");
alert(elm.innerHTML);
</script>
<p id="myp"> well this is darned interesting </p>
</body>
</html>

As you can see I’ve put the script before the p element is defined and the script is trying to access it . I thought I would get an error since p isn’t supposed to be defined yet but I got “well this is darned interesting” (the value of p) .
The same thing happens when I put the script in the head element. (testest with chrome and firefox).

Can anyone shed some light into what’s going on ?

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1 Answer

  1. Editorial Team

    Putting aside the specifics of what’s going on with this particular example, I’ll summarize the general principle. My source is Flanagan, JavaScript: The Ultimate Guide, 5/e.

    It’s too simplistic to think of a browser as reading a line of HTML and rendering something in response. Certainly it scans the input sequentially, but the sequence of DOM object construction is not line-for-line with the scanned input. Better to treat it as a black box.

    If you want to run a script that will manipulate DOM elements, the safe and sure way is to run it as a handler for the onload event. Sometimes you can write inline script to do it, but this is a gray area. Flanagan writes:

    Most browsers seem to allow scripts to manipulate any document
    elements that appear before the <script> tag. Some JavaScript
    programmers do this routinely. However, no standard required it to
    work, and there is a persistent, if vague, belief among some
    experienced JavaScript coders that placing document manipulation code
    within <script> tags can cause problems…. (p. 256)

    However, you may need to manipulate the DOM after all the HTML has been parsed but before all images are loaded. If that’s your need, then a tool like jQuery and its ready event is the way to go.

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