functional.js

functional.js

A lightweight, TypeScript-first functional programming library

Features

• TypeScript-first with excellent type inference
• Lightweight (~3KB gzipped) with strong tree-shaking
• Auto-curried functions for point-free programming
• Zero dependencies
• Modern ESM + CJS dual package
• Full test coverage (100%)

Install

npm install functional.js # or
pnpm add functional.js # or
yarn add functional.js

Quick Start

import { pipe, map, filter, reduce } from "functional.js";

const data = [1, 2, 3, 4, 5, 6];

const result = pipe(
    data,
    filter((x) => x % 2 === 0),
    map((x) => x * 2),
    reduce((acc, x) => acc + x)
);

console.log(result); 

Using Curry

import { curry } from "functional.js";

const add = curry((a: number, b: number) => a + b);

add(1, 2);
add(1)(2);

const add5 = add(5);
add5(10);

Point-Free Style

import { flow, map, filter, pluck } from "functional.js";

interface User {
    name: string;
    age: number;
    active: boolean;
}

const getActiveUserNames = flow(
    filter<User>((u) => u.active),
    map((u) => u.name.toUpperCase())
);

const users: User[] = [
    { name: "Alice", age: 30, active: true },
    { name: "Bob", age: 25, active: false },
    { name: "Charlie", age: 35, active: true }
];

console.log(getActiveUserNames(users));

Core Functions

Function Composition

• curry(fn) — Auto-curry any function
• compose(...fns) — Right-to-left function composition
• pipe(value, ...fns) — Left-to-right data pipeline
• flow(...fns) — Left-to-right function composition

Array Operations

• map(fn, array) — Transform array elements
• filter(fn, array) — Filter array by predicate
• reduce(fn, array) — Reduce array (no initial value)
• fold(fn, initial, array) — Fold array with initial value
• flatMap(fn, array) — Map and flatten
• each(fn, array) — Iterate over array
• partition(fn, array) — Split array by predicate
• group(fn, array) — Group array by key function
• zip(arr1, arr2) — Zip two arrays together
• zipWith(fn, arr1, arr2) — Zip with a combining function
• uniq(array) — Remove duplicates
• uniqBy(fn, array) — Remove duplicates by key function

Array Queries

• first(fn, array) — Find first matching element
• last(fn, array) — Find last matching element
• every(fn, array) — Check if all elements match
• any(fn, array) — Check if any element matches
• best(comparator, array) — Find best element by comparator

Object Operations

• prop(key) — Create property accessor
• pluck(key, array) — Extract property from array of objects
• pick(keys, obj) — Select properties from object
• omit(keys, obj) — Remove properties from object
• path(pathArray, obj) — Get nested property value
• assoc(key, value, obj) — Set property immutably
• dissoc(key, obj) — Remove property immutably
• assign(obj1, obj2) — Merge objects

Utilities

• identity(x) — Return input unchanged
• constant(x) — Create constant function
• tap(fn) — Execute side effect and return input

Type Checks

• isFunction, isObject, isArray
• isString, isNumber, isDate, isRegExp
• exists, truthy, falsy

TypeScript Support

All functions have full TypeScript support with proper type inference:

import { pipe, map, filter } from "functional.js";

const numbers = [1, 2, 3, 4, 5];

const result = pipe(
    numbers,
    filter((x) => x > 2),
    map((x) => x.toString())
);

Comparison

Library	Bundle Size	TypeScript	Auto-Curry	Notes
functional.js	~3KB	Strong	Yes	Data-last, zero dependencies
Ramda	~50KB	Limited	Yes	Data-last, large API
lodash/fp	~24KB	Good	Yes	Data-last wrappers over lodash
underscore	~17KB	Limited	No	Data-first utilities
fp-ts	~15KB	Strong	No	Types-first, higher learning curve

Performance

Benchmarks run on Node.js 24.13.0 (macOS), using small/medium/large scenarios with isolated processes, warmups, and median results.

Highlights from the latest run:

• functional.js wins 31 of 45 ops/sec metrics
• functional.js is fastest cold start at 0.81 ms
• functional.js leads 4 of 9 memory benchmarks (lowest heap delta)
• functional.js is up to 3.86x faster than the runner-up when it leads (1.57x average)

Using AI Assistants

Use these prompts with AI assistants (ChatGPT, Claude, Cursor, etc.) to get accurate functional.js help:

For learning

• "Show me how to use pipe in functional.js with TypeScript"
• "Compare functional.js pipe with Ramda pipe"
• "Explain when to use flow versus compose in functional.js"
• "Give me a functional.js example for transforming an array of users"

For migration

• "Convert this Ramda code to functional.js: [paste code]"
• "Migrate lodash/fp chain to functional.js pipe: [paste code]"
• "Replace compose with flow using functional.js: [paste code]"

For problem-solving

• "How do I transform an array of users with functional.js"
• "Show me point-free style with functional.js"
• "Build a reusable pipeline with flow and use it on data"

For debugging

• "Why does reduce throw on this input in functional.js: [paste code]"
• "My curried function returns another function, what did I do wrong in functional.js: [paste code]"

License

MIT License © Lee Crossley

Acknowledgments

Inspired by Ramda, lodash/fp, and fp-ts. Built for modern TypeScript development.