RSLint - A extremely fast JavaScript linter

ESLint is a de facto solution for linting JavaScript code. Also, C is the first thing that comes to our minds when we talk about compile some code of every kind. I know that a lot of compilers are compiled using the language that they compile 😕. But when we talk about parsers, abstract syntax trees (AST), and compile to binary: C comes to our minds. Maybe I'm old, I don't know. 🤔

And, in this post I'll talk about other linters for JavaScript that are growing around nowadays.

The Deno alternative

Deno has re-thought the Node.js world with a lot of really great decisions: a built-in bundler, documentation generator, a code formatter, and 🥁... deno_lint, a linter made in Rust 🦾.

Why Rust 🦀? Well, it's a powerful language: thread-safe and memory safe without a garbage collector. We can expect better speed than a statically typed language with a performance similar to C. And this includes more speed than JavaScript for an intensive CPU usage task: to parse and analyze code.

deno lint

The deno_lint tries to support the recommended set of rules from ESLint and TypeScript out of the box. But, this still doesn't support JSX code, so it's not easy to use with our React projects. 😞

You can try it by using npm install @node-rs/deno-lint in your nodejs project.

RSLint

Another linter appeared and is called RSLint, but it's only for JavaScript 🙃. This project is still in early development, that means it isn't ready for production yet.

RSLint uses rowan, a Rust library for syntax trees that was developed for Rust analyzer. Rowan models trees are immutable syntax trees, instead of mutable AST that are expensive to clone.

RSLint only implements around 25 rules nowadays, but the implementation of ESLint recommended rules and support for JSX is in the roadmap.

Additionally, RSLint will be available as an npm package with a pre-built binary as well.

Can we say this is better?

Not at the moment but, due to the immutability of the trees, the highly parallelization is a fact: «files linted in parallel, rules run in parallel, nodes could be traversed in parallel in the future» said the README.

The Rust like errors is a big different with the ESLint output, sometimes is not very friendly and you should web search which means the violation of a specific rule. If RSLint parse this code:

if true {
  /* */
} else {
  /* */
}

We can get this output:

error[SyntaxError]: Expected token `L_PAREN` but instead found `TRUE_KW`
  ┌─ tests\main.js:1:4
  │
1 │ if true {
  │    ^^^^ Unexpected

error[SyntaxError]: Expected token `R_PAREN` but instead found `L_CURLY`
  ┌─ tests\main.js:1:9
  │
1 │ if true {
  │         ^ Unexpected

error[no-constant-condition]: Unexpected constant condition
  ┌─ tests\main.js:1:4
  │  
1 │   if true {
  │      ^^^^ this condition is always truthy...
2 │     /* */
3 │   } else {
  │ ┌────────'
4 │ │   /* */
5 │ │ }
  │ └─' ...which makes this unreachable

💡 Wait, can RSLint parse JavaScript with invalid syntax? Yes!

The error recovery capabilities of RSLint refers to a parser being able to take in incorrect source code, and still parse a mostly correct AST out of it. Most linters do not attempt this at all. For example, ESLint and deno_lint's parsers make no attempt at recovery. When the parsers encounter an error they return an error result and quit parsing, producing no AST.

This means it's impossible for the linters to lint wrong code, which is an amazing feature for on-the-fly linting in things such as IDEs.

And this could be the most amazing feature that RSLint can bring to us. 🎉