What it is

Effect is a TypeScript library for building robust, maintainable, type-safe, and production-grade applications. It lives in the TypeScript and JavaScript ecosystem, distributed as the core effect package alongside a monorepo of integration packages that extend it. The project is MIT-licensed, has been developed for roughly seven years, and currently ships Effect V4 as a release candidate, with the main branch holding v4 development and the v3 branch retaining the previous line.

The concrete problem Effect addresses is the difficulty of handling hard problems at scale in TypeScript: typed errors, dependency injection, structured concurrency, scheduling, tracing, and unified schema validation. Rather than assembling separate libraries for each concern, Effect provides these capabilities through one cohesive runtime and type system. Integration packages extend the core to specific runtimes and data stores, including platform services for the browser, Bun, Deno, and Node.js, and SQL clients for ClickHouse, Cloudflare D1, libSQL, Microsoft SQL Server, MySQL, PostgreSQL, PGlite, and SQLite across several runtimes.

Key capabilities

• Typed errors that are tracked in the type system rather than thrown as untyped exceptions.
• Dependency injection for composing and substituting services across an application.
• Structured concurrency for managing concurrent and parallel work with defined lifetimes.
• Scheduling for controlling when and how effects execute.
• Tracing and observability, with OpenTelemetry listed among the project topics.
• Unified schema validation shared across the codebase.
• Clustering and CLI support, both listed among the project topics.

Who uses it and how

• Teams building production-grade TypeScript applications that need typed error handling and dependency injection in one runtime.
• Developers running Effect on Node.js, Bun, Deno, or in the browser through the corresponding @effect/platform-* packages.
• Applications connecting to SQL databases through the @effect/sql-* clients, including PostgreSQL, MySQL, ClickHouse, and Microsoft SQL Server.
• Edge and serverless deployments using the Cloudflare D1 and Cloudflare Durable Objects SQLite clients.
• Mobile and embedded scenarios using the SQLite client for React Native.

Getting started

Install the V4 release candidate with npm install effect@rc. The project requires TypeScript 5.9 or newer, Node.js 18 or newer as the general minimum, and the strict flag enabled in tsconfig.json; some integration packages require newer runtimes, such as @effect/sql-sqlite-node, which needs Node.js 22.16 or newer.

When to use it — and when not to

Effect suits teams that want typed errors, dependency injection, structured concurrency, and schema validation unified in one TypeScript runtime rather than stitched together from separate libraries. Adopters must meet the toolchain requirements: TypeScript 5.9 or newer, Node.js 18

npm install effect@rc