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

A working draft of the FunctionalScript specification.

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

The fjs CLI provides several commands:

| Command | Description | |------------------|-----------------------------------------------------------| | fjs test / t | Run the FunctionalScript test suite | | fjs compile / c | Compile a .f.ts module to JavaScript | | fjs run / r | Run a FunctionalScript module as a Node program | | fjs cas / s | Content-addressable storage (add, get, list) | | fjs mcp / m | Run an MCP server over stdio exposing the CAS as tools | | fjs ci / i | Generate the GitHub Actions CI workflow |

Content-Addressable Storage (CAS)

FunctionalScript ships a built-in CAS for storing and retrieving blobs by their cryptographic hash:

fjs cas add myfile.txt   # store a file, print its cBase32 hash
fjs cas get <hash>       # restore a blob by hash
fjs cas list             # list all stored hashes

Blobs are stored under ~/.cas/ and addressed by their SHA-256 hash encoded in cBase32.

MCP Server

The CAS is also exposed as an MCP server so LLM agents can read and write blobs without a shell:

# register with Claude CLI
claude mcp add cas -- npx functionalscript m

See fs/cas/mcp/README.md for details on the cas_add, cas_get, and cas_list tools.

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.