Eleventy: Debugging

While working on my Paired Shortcode for images I finally ran into a situation where trial-and-error wasn’t getting me anywhere, and I finally had to dive into the debugging hooks in Eleventy1.

Debugging in Eleventy has two major parts: declaring/invoking the debug instance in your code, and tweaking the command you use to build your site.

Setting up debug

Eleventy ships with support for the npm debug module. You can set it up like this in your .eleventy.js file:

const debug = require("debug")("markllobrera");

The second set of parentheses is your app name/identifier, and it gets prepended to your debug statements in the output. This allows you to filter your debug statements out of the noise.

Debug statements are just like console.log() statements, which makes sense. From the docs:

debug exposes a function; simply pass this function the name of your module, and it will return a decorated version of console.error for you to pass debug statements to.

So you can simply do something like this:

debug("captionMarkup: ", captionMarkup);

Which would give you output like this:

markllobrera captionMarkup:  <figcaption>Early-morning fog in Richwood, KY</figcaption>

Remember that app name? Well, there it is, right in front of any debug statements you’ve written.

Running in debug mode

So how do you get debug output to show up? Well, when running your build in the command line you can try:

DEBUG=Eleventy* npx @11ty/eleventy

But this is a little too verbose for me.

You can also try:

DEBUG=*Error* npx @11ty/eleventy

Which would filter for errors. This is a bit more useful. But if you just want your stuff, put your app name into the match pattern in the build command, like so:

DEBUG=*markllobrera* npx @11ty/eleventy

You can even run the build without actually writing files, saving some compilation time:

DEBUG=*markllobrera* npx @11ty/eleventy --dryrun

Or, if you prefer, you can also use --serve or --watch (this is what I do most of the time).


  1. This is, of course, the reverse order of advice I give every person new to programming, where I urge them to figure out a couple of ways to debug before they rush headlong into building things. But here we are.