llms.txt

# functional.js

Lightweight, TypeScript-first functional programming library. Data-last design, auto-curried utilities, tree-shakeable ESM + CJS, zero dependencies.

## Core Philosophy

- Data-last design for composition
- Auto-curried by default
- ~3KB gzipped, zero dependencies
- Pure, immutable-by-default utilities

## Positioning

- Simpler than fp-ts, better inference than Ramda
- More consistent TypeScript than lodash/fp
- Designed for modern build tooling

## Imports

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

Named exports only. A `fjs` namespace export is available when needed.

## Quick Reference

All functions with TypeScript signatures.

### Core Composition

- `curry(fn: (...args: any[]) => any): CurriedFunction`
- `compose(...fns: Array<(arg: any) => any>): (arg: any) => any`
- `pipe(value: T, ...fns: Array<(arg: any) => any>): any`
- `flow(...fns: Array<(arg: any) => any>): (arg: any) => any`

### Arrays

All are data-last and auto-curried where applicable.

- `each(fn: (value: T, index: number) => any, items: T[]): void`
- `map(fn: (value: T, index: number) => U, items: T[]): U[]`
- `fold(fn: (acc: U, value: T, index: number) => U, initial: U, items: T[]): U`
- `reduce(fn: (acc: T, value: T, index: number) => T, items: T[]): T`
- `clone(items: T[]): T[]`
- `first(fn: (value: T, index: number) => boolean, items: T[]): T | undefined`
- `rest(fn: (value: T, index: number) => boolean, items: T[]): T[]`
- `last(fn: (value: T, index: number) => boolean, items: T[]): T | undefined`
- `every(fn: (value: T, index: number) => boolean, items: T[]): boolean`
- `any(fn: (value: T, index: number) => boolean, items: T[]): boolean`
- `select(fn: (value: T, index: number) => boolean, items: T[]): T[]`
- `best(fn: (a: T, b: T) => boolean, items: T[]): T | undefined`
- `whilst(fn: (value: T, index: number) => boolean, items: T[]): T[]`
- `partition(fn: (value: T, index: number) => boolean, items: T[]): [T[], T[]]`
- `group(fn: (value: T) => K, items: T[]): Record<K, T[]>`
- `shuffle(items: T[]): T[]`
- `nub(fn: (a: T, b: T) => boolean, items: T[]): T[]`
- `strictEquals(a: T, b: T): boolean`

Aliases:

- `foldl` → `fold`
- `reducel` → `reduce`
- `foldll` → `reduce`
- `head` → `first`
- `take` → `first`
- `tail` → `rest`
- `drop` → `rest`
- `all` → `every`
- `contains` → `any`
- `filter` → `select`
- `unique` → `nub`
- `distinct` → `nub`

### Array Ops

- `flatMap(fn: (value: T, index: number) => U[], items: T[]): U[]`
- `chain` → `flatMap`
- `flatten(items: T[][]): T[]`
- `zip(arr1: T[], arr2: U[]): Array<[T, U]>`
- `zipWith(fn: (a: T, b: U) => R, arr1: T[], arr2: U[]): R[]`
- `uniq(items: T[]): T[]`
- `uniqBy(fn: (value: T) => K, items: T[]): T[]`

### Objects

- `toArray(obj: Record<string, any>): Array<[key, value]>`
- `apply(methodOrTuple: K | [K, ...args], items: T[]): any[]`
- `assign(obj1: T, obj2: U): U & T`
- `extend` → `assign`
- `prop(key: K): (obj: Record<K, any>) => any`
- `pluck(key: K, items: T[]): Array<T[K]>`
- `pick(keys: K[], obj: T): Pick<T, K>`
- `omit(keys: K[], obj: T): Omit<T, K>`
- `path(pathArray: Array<string | number>, obj: any): T | undefined`
- `assoc(key: K, value: T[K], obj: T): T`
- `dissoc(key: K, obj: T): Omit<T, K>`

### Utilities

- `identity(value: T): T`
- `constant(value: T): () => T`
- `tap(fn: (value: T) => void): (value: T) => T`

### Async

- `mapAsync(fn: (value: T, index: number) => Promise<U>, items: T[]): Promise<U[]>`
- `filterAsync(fn: (value: T, index: number) => Promise<boolean>, items: T[]): Promise<T[]>`
- `reduceAsync(fn: (acc: T, value: T, index: number) => Promise<T>, items: T[]): Promise<T>`
- `foldAsync(fn: (acc: U, value: T, index: number) => Promise<U>, initial: U, items: T[]): Promise<U>`
- `eachAsync(fn: (value: T, index: number) => Promise<void>, items: T[]): Promise<void>`
- `pipeAsync(value: T, ...fns: Array<(arg: any) => Promise<any>>): Promise<any>`
- `flowAsync(...fns: Array<(arg: any) => Promise<any>>): (arg: any) => Promise<any>`
- `composeAsync(...fns: Array<(arg: any) => Promise<any>>): (arg: any) => Promise<any>`

### Type Checks

- `isFunction`, `isObject`, `isArray`, `isArguments`
- `isDate`, `isNumber`, `isRegExp`, `isString`
- `exists`, `truthy`, `falsy`

### Types

- `Predicate<T>`, `Comparator<T>`, `Mapper<T, U>`, `Reducer<T, U>`
- `UnaryFn<T, U>`, `AnyFunction`
- `Curried2`, `Curried3`, `Curried4`, `CurriedFunction`

## Common Patterns

### Data pipeline

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

const result = pipe(
    data,
    filter((x) => x.active),
    map((x) => x.name),
    reduce((acc, name) => acc + name)
);
```

### Reusable flow

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

const getNames = flow(
    filter((x) => x.active),
    map((x) => x.name)
);
```

### Currying for reuse

```typescript
import { curry, map } from "functional.js";

const discount = curry((percentage: number, price: number) => price * (1 - percentage / 100));
const apply10 = discount(10);

const discounted = map(apply10, prices);
```

## Common Use Cases

### Transforming and filtering data

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

const names = pipe(
    users,
    filter((u) => u.active),
    map((u) => u.name)
);
```

### Shaping objects

```typescript
import { pipe, pick, assoc, path } from "functional.js";

const summary = pipe(
    profile,
    pick(["id", "name"]),
    assoc("city", path(["details", "city"], profile))
);
```

### Async pipelines

```typescript
import { pipeAsync, mapAsync, filterAsync } from "functional.js";

const run = async (ids: number[]) => {
    const passingIds = await pipeAsync(
        ids,
        mapAsync((id) => fetchScore(id)),
        filterAsync((score) => score >= 50)
    );

    return passingIds;
};
```

## Migration Guides

### From Ramda

- `R.pipe` → `pipe`
- `R.compose` → `compose`
- `R.map` → `map`
- `R.filter` → `filter`
- `R.reduce` → `reduce`
- `R.groupBy` → `group`
- `R.uniq` → `uniq`
- `R.prop` → `prop`

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

const result = pipe(
    items,
    filter((item) => item.ready),
    map((item) => item.id)
);
```

### From lodash/fp

- `fp.flow` → `flow`
- `fp.compose` → `compose`
- `fp.map` → `map`
- `fp.filter` → `filter`
- `fp.reduce` → `reduce`
- `fp.get` → `path`
- `fp.pick` → `pick`
- `fp.omit` → `omit`

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

const getReadyIds = flow(
    filter((item) => item.ready),
    map((item) => item.id)
);
```

## Performance Characteristics

- Small bundle size and tree-shakeable named exports
- Data-last API enables partial application without allocations
- Async utilities avoid intermediate arrays when chained in `pipeAsync`

## TypeScript Best Practices

- Rely on inference for callbacks in `map`, `filter`, `reduce`
- Add generics only when inference needs help
- Prefer `flow` for reusable pipelines and `pipe` for inline usage
- Avoid explicit types for intermediate values unless required

## Type System Guide

### When to Add Types

Prefer inference for callbacks, especially with `map`, `filter`, and `reduce`:

```typescript
const numbers = [1, 2, 3, 4, 5];
const doubled = map((x) => x * 2, numbers);
```

Add generics only when inference is insufficient:

```typescript
const getActiveNames = flow(
    filter<User>((u) => u.active),
    map((u) => u.name)
);
```

Use `flow` when you want a reusable typed pipeline that can be called multiple times.

## Composition Guide

### Using pipe

Use `pipe` when you have the data already and want to run a pipeline immediately:

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

const result = pipe(
    users,
    filter((u) => u.active),
    map((u) => u.name)
);
```

### Using flow

Use `flow` when you want a reusable function:

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

const getActiveNames = flow(
    filter((u) => u.active),
    map((u) => u.name)
);

const names1 = getActiveNames(users);
const names2 = getActiveNames(otherUsers);
```

### Using compose

Use `compose` for right-to-left composition (less common):

```typescript
import { compose, map, filter } from "functional.js";

const getActiveNames = compose(
    map((u) => u.name),
    filter((u) => u.active)
);
```

## Common Gotchas

- `reduce` throws on empty arrays without an initial value (use `fold` with an initial value instead)
- Most functions are curried and data-last, so partial application is expected
- `filter` is an alias of `select`, and `head` is an alias of `first`
- `assign` and `shuffle` are immutable and return new values (do not mutate inputs)
- Data comes LAST, not first (e.g., `map(fn, array)` not `map(array, fn)`)
- `curry` ignores extra arguments beyond the function's arity (standard currying behavior)

## Comparison Matrix

| Library       | Bundle Size | TypeScript | Auto-Curry | Learning Curve |
| ------------- | ----------- | ---------- | ---------- | -------------- |
| functional.js | ~3KB        | Strong     | Yes        | Low            |
| Ramda         | ~50KB       | Medium     | Yes        | Medium         |
| lodash/fp     | ~24KB       | Medium     | Yes        | Low            |
| fp-ts         | ~15KB       | Strong     | No         | High           |

## Notes

- `reduce` throws on empty arrays without an initial value
- Most functions are curried and data-last, enabling partial application
- Prefer `pipe` for immediate evaluation and `flow` for reusable pipelines