A small piece of HTML can change what a visitor sees or how the browser understands a page. In this lesson, you will explore dns-prefetch and modulepreload links using a short example that you can inspect and change.
The Idea in Plain English
Modulepreload lets the browser fetch and parse a JavaScript module before normal module discovery. It is intended for module graphs, not classic scripts.
You can test this using a small local HTML file. Change one part at a time and check the result in the browser before adding more code.
A Working Example
Read the sample once before copying it. Notice the element names and attribute values, then compare them with the explanation below.
<head>
<link rel="modulepreload" href="/js/dashboard.js">
<script type="module" src="/js/dashboard.js"></script>
</head>What Each Part Does
- The link element declares a relationship such as a stylesheet or alternate language.
- The script element can load code or hold data; the type attribute tells the browser how to treat its contents.
- The head holds page metadata, not the visible lesson content.
- rel describes the relationship between the current page and a linked resource.
- href is the URL or fragment destination of a link.
- type chooses the relevant kind of script, input, media or embedded object.
The browser can prepare the module; module code still executes according to the module loading rules.
Keep the example small while learning. Once it behaves as expected, try it inside a larger page and make sure the URLs, files and surrounding HTML are correct.
Try It Yourself
Try this next: add a modulepreload for an entry dependency. Then inspect the page and explain what changed.
Common Beginner Mistakes
- Using modulepreload on an unrelated script or expecting the hint by itself to execute JavaScript.
- Copying the code without checking whether its linked files or endpoints exist; sample addresses and file paths need real resources.
- Choosing a tag only because its default appearance looks right, without checking whether it describes the content correctly.
Key Takeaways
- Modulepreload lets the browser fetch and parse a JavaScript module before normal module discovery. It is intended for module graphs, not classic scripts.
- The browser can prepare the module; module code still executes according to the module loading rules.
- Test the markup in a browser, and check accessibility and real-world behavior where relevant.