Jordan Nielson

Demystifying Babel Plugins: A Debugging Story

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In previous posts in this series, Build Tools Demystified and Demystifying @babel/preset-env I introduced the ideas I wanted to cover and then dove into @babel/preset-env to see what we could learn. In this post, I'm going to do a little more general treatment of babel plugins, though the examples I use will largely be babel-plugin-styled-components and @babel/preset-env since those are the ones that I most commonly utilize in my projects.

As part of understanding more about babel plugins, it's important to realize that the amount of things you need to know to work with plugins is significantly lower than what you need to write plugins. For writing plugins, definitely check out the babel handbook on plugins but there isn't a similar handbook of instructions that you can follow to work with or understand an existing plugin. If it was written following the handbook then you should be at a pretty solid place, but I'm going to assume most people haven't read through that awesome (and long) handbook and roll with that.

Major Concepts

There's 2 major concepts involved in babel plugins: Abstract Syntax Trees (ASTs) and the Visitor Pattern. The basic idea of ASTs it to take the code that you write and parse it to just the important language constructs and generate a tree structure of "nodes" that allows you to traverse the "code" to manipulate it. In order to manipulate it, babel utilizes the Visitor pattern which basically allows plugins to be aware of all the node types, select ones they care about and do their thing to them, before passing it back to babel. The combination of these two ideas is incredibly powerful and unlocks the wide range of plugins that exist in the babel ecosystem. The two plugins I'm most familiar with are @babel/preset-env and babel-plugin-styled-components, as mentioned earlier. In @babel/preset-env there's a wide array of plugins that are enabled based off the targets that are passed to it, more on @babel/preset-env here. Since preset-env is a preset and not a plugin, strictly speaking, the rest of this post will focus on babel-plugin-styled-components and the experiences I've had with it.

Using a Plugin

In my experience there's usually 3 ways that a plugin is used:

  1. Part of a preset
  2. Directly to enable a language feature/syntax
  3. Directly to support a library/code style

In each of these cases your approach with the plugin usually varies. In the first case, you often don't know the plugin is even being used. Unless you dig into the preset source or dependencies you often have no idea what plugins are included in it, and what impact they're having on your output. In the second, you include a plugin because you want to enable some new feature or syntax to use in your codebase. Usually teams will evaluate these additions pretty strictly, because the more plugins you have the longer your build process will take (by nature of each plugin running on the entire tree, even if it only actually does something on specific nodes). Then with the third type you have plugins like babel-plugin-styled-components that enable cool features of the library like SSR. The reason to call out these different uses of plugins is it greatly impacts how you approach the plugin if you run into issues with it.

Issues when trying to use a Plugin

In my experience there's usually 3 kinds of issues you run into:

  1. Plugin not doing something you want
  2. Plugin doing something you didn't expect or don't want
  3. Plugin running at all

In these three cases the way you approach debugging is a little different. In each case, it's super useful to have a minimal reproduction you're working with. Minimal reproductions make problems easier to notice and trace, as well as can be submitted to projects that accept bug reports if you happen to determine it's actually an issue with the project. There are of course issues that don't occur in a smaller reproduction, but in the majority of cases I've seen there's benefits to trying to boil it down to the simplest case possible. Once you have a simple case, there's a few things you can check for to figure out what might be going on. They depend on what's happening of course, but let's run through some possibilities:

Plugin isn't doing something you want it to

The first, and usually easiest to notice case is where you a plugin that isn't doing something you'd expect. An example of this that I ran into was with babel-plugin-styled-components with the ssr option flipped to true still producing a className mismatch warning. This warning indicates that something isn't quite right in the build process, since the babel-plugin is designed to generate consistent classNames for the server and client renders of a component. First, I had to rule out that it wasn't an issue of the plugin not running at all, and that was pretty easy to do by digging into node_modules and adding a simple console.log statement. Then, after spending some time trying to get a minimal reproduction, the smallest I could get still involved my code, a shared component library, babel-plugin-styled-components and @babel/preset-env - not a very small reproduction if you ask me! But, as I turned things off and back on checking for the issue, I realized that my code wasn't actually a part of the issue since it was getting transformed correctly and not producing the warning if the shared library code wasn't present. At this point I knew that it wasn't an issue with the actual application, so I was able to strip away pretty much all the application and boil it down to the simplest Next.js SSR setup I could get with the simplest library component that the issue would happen to. After playing with the options of various plugins for a while, I realized that babel-plugin-styled-components wasn't actually visiting any of the nodes that are output from the shared library component. Since we pre-build the shared library components, it turns out that we weren't using babel-plugin-styled-components but we were using @babel/preset-env to build them. The targets (something I talked about at length in my post on preset-env) were setup to have @babel/preset-env transform the template literals that babel-plugin-styled-components looks for and made it so that babel-plugin-styled-components didn't recognize the styled-components out of the shared library. This meant that it wasn't doing what I wanted it to, transform the code to be SSR compatible, because another plugin had already transformed the code previously. In this case, I learned that the order matters for babel plugins, since it's possible for one plugin to transform a node type into a different one and cause other plugins expecting the original type to not work.

Plugin is doing something that you don't want

Related to the previously discussed issue of a plugin not doing what you want, but significantly harder to notice in most cases, are plugins doing something that you don't want. I was recently involved in debugging an issue on the storybookjs repo where someone was running into an _esmodule property being added to their exported object by @babel/preset-env, something they weren't aware was happening until it started showing up in their storybook. In this case, they had forgotten to switch and tell @babel/preset-env to avoid transforming modules, and so it was transforming them. In most cases that will work just fine, but it does cause issues with other tools like webpack (for application code) being unable to use features like tree-shaking. As such, they had spent a couple hours trying to debug why this unexpected thing was happening and it boiled down to a plugin option. In this example, the plugin was doing something they didn't want anymore, but it took some prior knowledge on my part to know that _esmodule is a property used by @babel/preset-env to indicate that it is a transformed esModule file. In the cases of plugins doing unexpected things that you don't know about, the best way to recognize it that I've found is to occasionally glance at your output from the babel process (before minification/uglification ideally) to see if there's anything in there that you wouldn't expect given the options and targets that you've used. I find that checking on that a couple times a year is sufficient, since in most cases your targets and options don't need to change, but if you never check it then there's a pretty good chance you have a plugin doing something that you don't know about. On the bright side, issues of this type indicate that the plugin is actually running (although it might not be configured properly) so you can rule out the third type of issue!

Plugin is not running at all

Sometimes it's hard to tell if you have a plugin that isn't running or a plugin that isn't doing something you want, but in most cases it's usually pretty easy to debug and see if a plugin is being run. Since this is JavaScript and we use npm or yarn for our packages, you can usually go into the node_modules folder and edit the built library/plugin code to check and see if it's running - especially useful if it doesn't provide a debug option (which @babel/preset-env provides). In the cases of plugins not running, usually it's a config issue or picking the wrong plugin. I talked briefly in the overview post about the difference between transform and syntax plugins since one will actually adjust the output code while the other enables the parser to recognize the input. I also noted that transform plugins are supposed to enable the required syntax plugins for you. So, in most cases that I've seen issues of this type the config is the place to look and make sure there aren't plugins running before that are causing the plugin you want to not run, or to check and make sure that the options you pass the plugin are valid and correct.


Overall, babel plugins from a users perspective are quite simple. They're designed to take code and transform it into other code based on it's defined use case, options, and the targets of the build. These simple plugins have been built up into presets that enable you to turn on whole lists of plugins at once, and it's entirely possible (and relatively simple) to build your own preset! It could be a "smart" preset like @babel/preset-env or it could be a simple preset that enables the various plugins you need in your project.

Thanks for reading! I hope you've learned something! If you have, or if you have any comments or suggestions feel free to reach out and let me know. Thanks!

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