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1.2.0 • Public • Published



(graphic by @flipty)

ES5 (.js) version

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ES6 (.mjs) version

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Who—er, what—is dnstradamus?

dnstradamus is a very small script that uses dns-prefetch to perform early DNS lookups links to external sites. It uses IntersectionObserver to monitor <a> elements as they are scrolled into the viewport, as well as requestIdleCallback to take advantage of idle browser time to minimize jank. dnstradamus is inspired by quicklink, which performs link prefetching rather than DNS lookups.

Bah, why bother with DNS prefetching?

DNS lookup times are undoubtedly reduced by redundant DNS caches in your browser, operating system, and many layers beyond. Public DNS servers such as Cloudflare's and Google's public DNS also offer very fast resolution that help to make the web snappier for everyone everywhere. That said, sometimes DNS lookups can still take longer than we'd like.

A breakdown of response timings as shown in Chrome DevTools, with DNS lookup taking 92.27 ms in this example.

dns-prefetch gives us a resilient and fault-tolerant way to mask the latency of DNS lookups. Unfortunately, it may not be practical to manually add tons of these hints in your page's <head> to cover every single external link on your site. This is especially true for sites with lots dynamic or user-generated content. This means we're leaving an opportunity on the table to improve perceived navigation performance for outbound links.

Why not just prefetch pages, though?

Page prefetching goes a long way toward improving perceived performance. Yet, link prefetching can be wasteful, which is not always acceptable for people on metered data plans. quicklink mitigates this somewhat by checking to see if users have data saver mode enabled on Chrome for Android, as well as checking the Network Information API's effective connection type. Despite these mitigations, however, these signals are only available in Chrome.

On the other hand, speculative DNS lookups are low risk because of the relative cheapness of them compared to prefetching documents.

How to use dnstradamus

If you're the npm sort, you can install it:

npm install dnstradamus

From there, it's not much trouble to get up and running:

import dnstradamus from "dnstradamus";

document.addEventListener("DOMContentLoaded", function() {
  // Read below for more options
    observeChanges: true

If you'd prefer not to install with npm, there are scripts in both ES6 and ES5 flavors in the dist folder available in this repo as dnstradamus.min.mjs and dnstradamus.min.js, respectively.

However you use dnstradamus, you need to be aware that it depends on IntersectionObserver to work. If used in a browser that doesn't support IntersectionObserver, you can polyfill it conditionally with this script:

<script defer src=""></script>

If you don't load a polyfill for IntersectionObserver, dnstradamus will fail silently. This ensures that an unpolyfilled feature won't brick your website.


dnstradamus has a few options:


Default: "body"

The context in which links are selected. This option accepts any valid CSS selector. It's useful for helping you to narrow down where dnstradamus looks for links. For example, let's say you only wanted to do DNS prefetching for external links within an element with an ID of content:

  context: "#content"

This context should point to a unique element. If you're not sure if you should specify anything here, leaving it undefined is okay.


Default: (anchor, origin) => true

If you want to restrict what <a> elements dnstradamus prefetches DNS info for, include helps you to do that by providing a filter. This filter's interface includes the anchor element itself, as well as the origin it points to. From here, you can create your own filtering logic to determine what links should have DNS information prefetched for. Returning any expression that evaluates to true will include the link's origin for DNS prefetching. For example, let's say you wanted to exclude anchor elements with a data-nolookup attribute from DNS prefetching:

  include: (anchor, origin) => !("nolookup" in anchor.dataset)

Or, let's say you only wanted to include links that match a regex for specific domains:

  include: (anchor, origin) => /^https?:\/\/(www\.)?(alistapart\.com|css-tricks\.com)/i.test(origin)

The interface include provides should be flexible enough to let you figure out what's best for your application.


Default: false

Flipping this to true creates a MutationObserver to look for new links added to the DOM after initialization. This is useful for single page applications where large swaths of the DOM can change. When in doubt—or in settings where client-side routing isn't used—don't set this to true.

Note: The context option also restricts the scope of the mutation observer!

Special thanks

Thanks to my pal Dave (@flipty] for the neat graphic. Cheers, friend.

Thank you to BrowserStack for graciously providing free cross-platform browser testing services! BrowserStack


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