FunctionalScript

FunctionalScript is a safe, purely functional programming language and a strict subset of ECMAScript/JavaScript. It's inspired by

• JSON and JSON5 as subsets of JavaScript values. JSON is also a subset of FunctionalScript values.
• asm.JS (a precursor of WebAssembly), as a subset of JavaScript.
• TypeScript, as a superset of JavaScript.

The FunctionalScript specification describes the language the compiler accepts today; features not implemented yet are in spec/todo/.

Learn more about

• Purely Functional Programming in JavaScript,
• FunctionalScript and I/O.

This repository is a monorepo and distributed under MIT.

Getting Started

Install FunctionalScript via npm:

npm install -g functionalscript

or run the CLI without installing it, with npx functionalscript <command>.

Compiling a module

A FunctionalScript module is already a valid JavaScript module, so nothing has to be compiled in order to run it. The compiler serves the other direction: it evaluates a module and emits the data it exports, with every import resolved.

m.f.js:

export default "text"

input.f.js:

import c from "./m.f.js"
const a = 1
export default [a, a, c, { x: c }]

The output file extension picks the format:

fjs compile input.f.js output.f.js   # JavaScript
fjs compile input.f.js output.json   # JSON

output.f.js preserves the object graph — a value referenced more than once stays shared and is emitted as a const:

const c0 = "text"
export default [1,1,c0,{"x":c0}]

output.json is a tree, so shared values are expanded, and types that JSON cannot express (bigint, undefined) are not available:

[1,1,"text",{"x":"text"}]

The compiler currently accepts import statements, const declarations, and data expressions (objects, arrays, strings, numbers, bigint, booleans, null, undefined). Functions and computed expressions are not supported yet. See fjs/djs/README.md for the data language and its roadmap, and fjs/fsc/README.md for the compiler itself.

The fjs CLI

Command	Description	Documentation
fjs test	Run the FunctionalScript test suite	fjs/emergent_testing
fjs compile	Compile a FunctionalScript module to JavaScript or JSON	fjs/djs
fjs run	Run a FunctionalScript module as a Node program	fjs/README.md
fjs cas	Content-addressable storage (add, get, list)	fjs/cas/README.md
fjs mcp	MCP server over stdio, exposing the CAS and Evo as tools	fjs/mcp/README.md
fjs ci	Generate the GitHub Actions CI workflow	fjs/ci/README.md

Run fjs help to print the available commands, or see fjs/README.md for the full CLI reference. Commands also accept short aliases, which fjs help prints; the documentation spells them out instead.

Vision

We aim to create a safe, cross-platform programming language that can work in any JS platform without any build step. There are thousands of programming languages, and we don't want to create another one that others must learn. Instead, we take the opposite approach: we remove everything that makes the most popular and cross-platform language unsafe, insecure, or less portable.

Applications

FunctionalScript code can be used:

• safely in any JavaScript/TypeScript application or library;
• as a JSON with expressions, see DJS;
• as a query language;
• as a smart contract programming language in DeFi.

Design Principles

In FunctionalScript:

• Any module is a valid JavaScript module. No additional build steps are required.
• Code should not have side-effects. Any JavaScript statement, expression, or function that has a side effect is not allowed in FunctionalScript. There are no exceptions to this rule, such as unsafe code, which can be found in Rust, C#, and other languages.
• A module can depend only on another FunctionalScript module.
• It also has no standard library. Only a safe subset of standard JavaScript API can be used without referencing other modules.

Sponsors

• KirillOsenkov,
• antkmsft,
• Mark Heyman.