Any linter to warn about side-effects in JavaScript?

Question

With the flexibility of JavaScript, we can write code full of side-effects, or just purely functional.

I have been interested in functional JavaScript, and wanting to start a project in this paradigm. And a linter about that can surely help me gathering good practices. Is there any linter to enforce pure functional and side-effect free style?

Answer 1

Purity Analysis is equivalent to Solving the Halting Problem, so any kind of static analysis that can determine whether code is pure or impure is impossible in the general case. There will always be infinitely many programs for which it is undecidable whether or not they are pure; some of those programs will be pure, some impure.

Now, you deliberately used the term "linter" instead of static analyzer (although of course a linter is just a static analyzer), which seems to imply that you are fine with an approximate heuristic result. You can have a linter that will sometimes tell you that your code is pure, sometimes tell you that your code is impure, and most times tell you that it cannot decide whether your code is pure or impure. And you can have a whitelist of operations that are known to be pure (e.g. adding two Numbers using the + operator), and a blacklist of operations that are known to be impure (e.g. anything that can throw an exception, any sort of loops, if statements, Array.prototype.forEach) and do a heuristic scan for those.

But in the end, the results will be too unreliable to do anything serious with them.

Answer 2

I haven't used this myself but I found this plugin for ESLint: https://github.com/jfmengels/eslint-plugin-fp