Klubok

Do-notation pipes for Promise-based or pure functions with easy mocking.

Inspired by fp-ts/Effect bind Do-notation, but smaller and simpler. Primarily created for easy mocking of functions, allowing you to write extensive unit tests using the London school approach.

Features

• 🔄 Sequential composition — Chain pure and async functions with automatic context passing
• 🧪 Easy mocking — Mock any step in the pipeline without complex dependency injection
• 🎯 Selective execution — Run only specific functions using the only parameter
• 🛡️ Type-safe — Full TypeScript support with inferred types for up to 20 functions
• 🐛 Debug-friendly — Automatic context attachment to errors for easier debugging
• ⚡ Mutable state — Support for mutable operations when needed

Installation

npm install klubok

Quick Start

import { pure, eff, klubok } from 'klubok'

const catsBirthdays = klubok(
  eff('cats', async ({ ids }: { ids: number[] }) => {
    /* fetch cats from DB */
  }),

  pure('catsOneYearOld', ({ cats }) =>
    cats.map(cat => ({ ...cat, age: cat.age + 1 }))
  ),

  eff('saved', async ({ catsOneYearOld }) => {
    /* save to DB */
  })
)

// Production usage
catsBirthdays({ ids: [1, 2, 3] })

// In tests: mock 'cats' and run only 'catsOneYearOld'
catsBirthdays(
  { ids: [1, 2, 3] },
  {
    // DB response mock
    cats: () => [
      { name: 'Barsik', age: 10 },
      { name: 'Marfa', age: 7 }
    ]
  },
  // call only this function
  ['catsOneYearOld']
)
// Promise<{ cats: [...], catsOneYearOld: [{ name: 'Barsik', age: 11 }, ...] }>

API Reference

pure<K, C, R>(key: K, fn: (ctx: C) => R)

Creates a synchronous keyed function for the pipeline.

import { pure, klubok } from 'klubok'

const fn = klubok(
  pure('inc', (ctx: { number: number }) => ctx.number + 1),
  pure('str', ({ inc }) => inc.toString())
)

await fn({ number: 1 })
// { number: 1, inc: 2, str: '2' }

eff<K, C, R>(key: K, fn: (ctx: C) => Promise<R>)

Creates an asynchronous (Promise-based) keyed function for the pipeline.

import { eff, klubok } from 'klubok'

const fn = klubok(
  eff('fetchUser', async ({ id }: { id: number }) => {
    return { id, name: 'Alice' }
  }),
  eff('fetchPosts', async ({ user }) => {
    return [{ title: 'Hello' }]
  })
)

await fn({ id: 1 })
// { id: 1, user: {...}, posts: [...] }

mut(fn: KeyedFunction)

Marks a function as mutable, allowing it to override a previous result with the same key.

import { pure, mut, klubok } from 'klubok'

const fn = klubok(
  pure('data', () => [1, 2, 3]),
  mut(pure('data', ({ data }) => data.map(x => x * 2)))
)

await fn({})
// { data: [2, 4, 6] }

klubok(...fns)

Main function that composes keyed functions into a pipeline. Returns a function with the signature:

(
  ctx: C,
  mock?: { [key: string]: value | ((ctx) => value | Promise<value>) },
  only?: string[]
) => Promise<{ ...ctx, ...results }>

Parameters:

Parameter	Type	Description
ctx	object	Initial context (input data)
mock	object (optional)	Mock implementations for any step. Values can be static or functions
only	string[] (optional)	Execute only specified keys, skipping others

Mocking

Static Mocks

Replace a function's result with a static value:

const fn = klubok(
  eff('fetchData', async () => {
    /* real API call */
  }),
  pure('process', ({ fetchData }) => fetchData.map(x => x * 2))
)

await fn({}, { fetchData: [1, 2, 3] })
// { fetchData: [1, 2, 3], process: [2, 4, 6] }

Function Mocks

Replace a function with a custom implementation (sync or async):

await fn(
  {},
  {
    fetchData: () => [1, 2, 3],
    process: ({ fetchData }) => fetchData.filter(x => x > 1)
  }
)

Mocks with Context Access

Mock functions receive the accumulated context:

await fn(
  { userId: 42 },
  {
    fetchData: ({ userId }) => {
      console.log('Mock called with userId:', userId)
      return [{ id: userId, value: 100 }]
    }
  }
)

Selective Execution (only)

Run only specific functions in the pipeline:

const fn = klubok(
  eff('step1', async () => { /* ... */ }),
  pure('step2', ({ step1 }) => step1 * 2),
  eff('step3', async ({ step2 }) => { /* ... */ })
)

// Execute only step1 and step2
await fn({}, {}, ['step1', 'step2'])
// { step1: ..., step2: ... } — step3 is skipped

Useful for unit testing individual transformations without running the entire pipeline.

Error Handling

Automatic Context Attachment

When an error is thrown, Klubok automatically attaches the current context to the error stack:

const fn = klubok(
  pure('step1', () => 42),
  eff('step2', async ({ step1 }) => {
    throw new Error('Failed!')
  })
)

try {
  await fn({})
} catch (e) {
  console.log(e.stack)
  // Error: Failed!
  //   at ...
  // context: { step1: 42 }
}

Approved Errors

Errors with isApproved = true property won't have context attached (useful for expected business logic errors):

class ApprovedError extends Error {
  isApproved = true
  constructor(message: string, options?: ErrorOptions) {
    super(message, options)
  }
}

const fn = klubok(
  eff('validate', async () => {
    throw new ApprovedError('Invalid input')
  })
)

Type Safety

Klubok provides full TypeScript inference for pipelines up to 20 functions. Each step's output type is automatically added to the context type for subsequent steps:

const fn = klubok(
  pure('a', (ctx: { x: number }) => ctx.x + 1),        // ctx: { x: number }
  pure('b', ({ a }) => a.toString()),                  // ctx: { x: number, a: number }
  pure('c', ({ a, b }) => a + b.length)                // ctx: { x: number, a: number, b: string }
)
// Result: Promise<{ x: number, a: number, b: string, c: number }>

Use Cases

Testing with London School Approach

Mock external dependencies at any level without changing production code:

// production.ts
export const processOrder = klubok(
  eff('fetchOrder', async ({ orderId }) => db.orders.find(orderId)),
  eff('validateStock', async ({ fetchOrder }) => warehouse.check(fetchOrder)),
  eff('chargePayment', async ({ fetchOrder }) => paymentGateway.charge(fetchOrder)),
  eff('shipItems', async ({ fetchOrder, chargePayment }) => shipping.create(fetchOrder))
)

// test.ts
await processOrder(
  { orderId: 123 },
  {
    fetchOrder: () => ({ id: 123, items: ['item1'] }),
    validateStock: () => true
  },
  ['chargePayment'] // Test only payment logic
)

Data Transformation Pipelines

Chain pure transformations with clear, testable steps:

const transformData = klubok(
  pure('parsed', ({ raw }: { raw: string }) => JSON.parse(raw)),
  pure('validated', ({ parsed }) => schema.parse(parsed)),
  pure('normalized', ({ validated }) => normalize(validated)),
  pure('enriched', ({ normalized }) => addMetadata(normalized))
)

See Also

• Klubok-Gleam — Gleam implementation of Klubok

License

MIT © Vladislav Botvin