Docs / Language Manual / Introduction

You are currently looking at the v8.0.0 docs (Reason v3.6 syntax edition). You can find the latest manual page here.

ReScript

ReScript is the language for folks who don't necessarily love JavaScript, but who still acknowledge its importance.

ReScript looks like JavaScript, works like JavaScript, works with JavaScript, and compiles to the highest quality of clean, readable and performant JavaScript, directly runnable in the browser and Node.js.

Difference vs TypeScript

We respect TypeScript very much and think that it's a positive force in the JavaScript ecosystem. ReScript shares some of the same goals as TypeScript, but is different enough regarding some important nuances:

  • TypeScript's (admittedly noble) goal is to cover the entire JavaScript feature set and more. ReScript covers only a curated subset of JavaScript. For example, we emphasize plain data + functions over classes, clean pattern matching over fragile ifs and dynamic object dispatches, proper data modeling over string abuse, etc. JavaScript supersets will only grow larger over time; ReScript doesn't. *

  • Consequently, TypeScript's type system is necessarily complex, pitfalls-ridden, potentially requires tweaking, sometime slow, and requires quite a bit of noisy annotations that often feel like manual bookkeeping rather than clear documentation. In contrast, ReScript's type system:

    • Is deliberately curated to be a simple subset most folks will have an easier time to use.

    • Has no pitfalls, aka the type system is "sound" (the types will always be correct). E.g. If your value is not null, it'll never lie and slip through some undefined value. ReScript code has no null errors.

    • Is the same for everyone. No knobs, no bikeshedding opportunity.

    • Runs extremely fast precisely thanks to its simplicity and curation. We're one of the fastest compiler & build system for JavaScript development.

    • Doesn't need type annotations. Annotate as much or as little as you'd like. The types are inferred by the language (and, again, are guaranteed correct).

  • Migrating to TypeScript is done "breadth-first", whereas migrating to ReScript is done "depth-first". You can convert your codebase to TypeScript by "turning it on" for all files and annotate here and there; but how much type safety did you gain? How do you measure it? Type errors can still slip in and out of the converted pieces. For ReScript, our interop features draws clear boundaries: there's pure ReScript code, and there's JS interop code. Every piece of converted ReScript code is 100% clean. You'd convert file by file and each conversion increases your safety monotonically.

* When you absolutely need to write or interoperate with free-for-all JavaScript, we expose enough escape hatches for you.

Other Highlights

Aside from the aforementioned simple, robust and fast type system, ReScript presents a few more advantages.

Faster than JavaScript

JavaScript's been aggressively optimized by talented engineers over a long span. Unfortunately, even for seasoned JS devs, it can be hard to know how to properly leverage JS' performance. ReScript's type system and compiler naturally guides you toward writing code that's very often performant by default, with good leverage of various Just-In-Time optimizations (hidden classes, inline caching, avoiding deopts, etc.).

A widespread adage to write fast JavaScript code is to write as if there's a type system (in order to trigger JS engines' good optimization heuristics); ReScript gives you a real one and generates code that's friendly to optimizations by default.

High Quality Dead Code Elimination

The JavaScript ecosystem is very reliant on dependencies. Shipping the final product inevitably drags in a huge amount of code, lots of which the project doesn't actually use. These regions of dead code impact loading, parsing and interpretation speed. ReScript provides powerful dead code elimination at all levels:

  • Function and module level code elimination is facilitated by the well-engineered type system and purity analysis.

  • At the global level, ReScript generates code that are naturally friendly to dead code elimination done by bundling tools such as Rollup and Closure Compiler, after its own sophisticated elimination pass.

  • The same applies for ReScript's own tiny runtime (which is written in ReScript itself).

Tiny JS Output

A Hello world ReScript program generates 20 bytes JS code. Additionally, the standard library pieces you require in are only include when needed.

Fast Iteration Loop

ReScript's build time is one or two orders of magnitude faster than alternatives. In its watcher mode, the build system usually finishes before you switch screen from the editor to the terminal tab (two digits of milliseconds). A fast iteration cycle reduces need of keeping one's mental state around longer; this in turns allows one to stay in the flow longer and more often.

Readable Output & Great Interop

Unreadable JavaScript code generated from other compiled-to-js languages makes it so that it could be, practically speaking:

  • Hard to debug (cryptic stack trace, mangled variable names)

  • Hard to learn from (non-straightforward mapping of concepts from one language to another)

  • Hard to profile for performance (unclear what runtime performance cost there is)

  • Hard to integrate with existing hand-written JS code

ReScript's JS output is very readable. This is especially important while learning, where users might want to understand how the code's compiled, and to audit for bugs.

This characteristic, combined with a fully-featured JS interop system, allows ReScript code to be inserted into an existing JavaScript codebase almost unnoticed.

Preservation of Code Structure

ReScript maps one source file to one JavaScript output file. This eases the integration of existing tools such as bundlers and test runners. You can even start writing a single file without much change your build setup. Each file's code structure is approximately preserved, too.

Conclusion

We hope the above gave you enough of an idea of ReScript and its differentiators. Feel free to try it online to get a feel!