Effect-TS Has a Free API That Brings Typed Functional Effects to TypeScript

Effect is a TypeScript library for building complex, reliable applications. It brings typed errors, dependency injection, concurrency, and observability — all type-safe.

Quick Start

npm install effect

Why Effect?

Regular TypeScript:

// What can go wrong? What does this need? No idea from the type.
async function getUser(id: string): Promise<User> { ... }

With Effect:

// Types tell you: can fail with NotFound or DbError, needs DbClient
function getUser(id: string): Effect<User, NotFound | DbError, DbClient> { ... }

Basic Usage

import { Effect, pipe } from 'effect'

const program = pipe(
  Effect.succeed(42),
  Effect.map(n => n * 2),
  Effect.tap(n => Effect.log(\`Result: \${n}\`))
)

Effect.runPromise(program) // logs "Result: 84"

Typed Errors

class NotFound { readonly _tag = 'NotFound' }
class DbError { readonly _tag = 'DbError' }

const getUser = (id: string): Effect.Effect<User, NotFound | DbError> =>
  pipe(
    queryDb(id),
    Effect.catchTag('DbError', () => Effect.fail(new NotFound()))
  )

// TypeScript knows EXACTLY which errors can occur

Dependency Injection (Services)

class DbClient extends Context.Tag('DbClient')<DbClient, { query: (sql: string) => Effect.Effect<any[]> }>() {}

const program = pipe(
  DbClient,
  Effect.flatMap(db => db.query('SELECT * FROM users'))
)

// Provide the implementation at the edge
const runnable = program.pipe(
  Effect.provideService(DbClient, { query: realDbQuery })
)

Concurrency

// Run 3 effects concurrently, collect results
const results = Effect.all([fetchUsers, fetchPosts, fetchComments], { concurrency: 3 })

// Race — first to complete wins
const fastest = Effect.race(fetchFromCacheEffect, fetchFromDbEffect)

The Bottom Line

Effect is for TypeScript what ZIO is for Scala. If you build complex backend systems and want typed errors, DI, and concurrency — Effect is the most complete solution in the TS ecosystem.

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