Create a codemod - Part 1 - Why?

UpdatedSeptember 14, 2022

Royston Shufflebotham is a Front-end developer and architect at i2, in Cambridge, UK.

He got his first taste of coding in 1981, and really hasn't stopped since then, working his way through backend and frontend jobs in text matching, web & PKI infrastructure and crypto, finance, web server software, laboratory automation and investigative analysis, journeying through Quick Basic, Perl, TCL, C++ (several times), Delphi, C#, Java (several times) and JavaScript.

He appears to have finally settled down as a Front-end developer, preferring to work in React and TypeScript.

He also has a very unhealthy fondness for fiddling round with, or writing, developer tools and DevOps instead of writing real production code.

NOTE: This is Royston's personal blog.

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Oh, and in case you're wondering, it's pronounced ˈrɔɪstən ˈʃʌfl̩boθəm (or ˈrɔɪstən ˈʃʌfl̩bəʊθəm if it's easier for you)(guide). Yeah, I'm definitely helping there...

So, you've got a nice big codebase full of code you're reasonably happy with.

Great!

But, of course, it's rotting.

If you've been coding with JavaScript (or most other languages, to be honest) for any amount of time, you'll have noticed that habits and best practices change over time. What was a cool way to do something six months ago isn't quite so cool any more:

maybe there's some new syntax in JS or a new Babel plugin
maybe there's a new library that makes it easier
maybe one of your libraries or frameworks has introduced a cool new feature

What's stopping you adopting those new features or practices? Most likely it's your existing codebase.

Maybe you want to turn on a new eslint rule across your codebase, but you currently have a lot of violations. You could mark them all as excluded so that you can apply it to new code, but that old code simply isn't going to fix itself.

You want to improve your codebase, but as you get more and more of it, it feels harder and harder to change it. So it starts to fall behind. That's the rot.

Codemod it!

A codemod (code mod-ification) is a tool that automatically applies a set of changes to an entire codebase. So that little thing you'd like to add to your codebase that would make everything better suddenly becomes something you could actually consider.

Should I write a codemod or just make the changes by hand?

This is a great question, and the excellent XKCD comic has addressed this on... several... occasions.

It's a simple equation:

How long will it take you to develop the codemod? (Let's call that time D.)

Note that, like any code, getting something simple and basic working won't take very long, but making it cope with all the edge cases once it hits the reality of your codebase might increase this time.
How long will it take to make the changes manually, without the codemod? (Let's call that time M.)

Note that if this is an error-prone modification to make, the cost will be higher.

Is M > D? If so, the codemod will be worthwhile. If not, it's probably not going to be worth your time.

Writing a codemod might feel cool, but if you're only going to run it once, you either need to be able to write codemods quickly or your codebase had better be large enough - or you need to share your codemod with enough other people - to make it worthwhile.

So, as you can tell, the more efficiently you can write a codemod, the more value you'll get out of writing codemods.

That's why I've written this series of articles.

Summary

We've reviewed what a codemod is, why you might (or might not) want to write one. In part 2, we'll start to look into what they involve. Until next time!

Create a codemod - Part 1 - Why?

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