Effect-ts for TypeScript: Complete Guide with Examples

What is Effect-ts and why is it needed?

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Effect-ts is a powerful library for TypeScript that brings functional programming concepts to the language, allowing you to write clean, composable, and type-safe code. It solves one of the main problems in TypeScript (and JavaScript in general) development — managing side effects.

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In traditional imperative programming, side effects (working with networks, files, databases, generating random numbers, working with time) are scattered throughout the code, making it difficult to test, debug, and analyze. Effect-ts offers an elegant approach: all side effects are described as data (values) that can be combined, transformed, and executed in a controlled environment.

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Main advantages of Effect-ts:

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• Type safety: The TypeScript compiler checks error types and successful results at the type level.
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• Composability: Effects are easily combined using functions like map, flatMap, zip, and others.
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• Error handling: Errors are handled as part of the effect type, not through exceptions.
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• Testability: Dependencies (e.g., HTTP clients, databases) can be easily substituted.
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• Concurrency: Built-in support for asynchrony, parallelism, and operation cancellation.
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• Predictability: Code becomes more declarative and easier to understand.
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Effect-ts is especially useful in large projects where reliability, testability, and scalability are important. It is used in backend development (Node.js), frontend (React), and even in mobile applications.

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Installation

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To install Effect-ts in your project, use npm or yarn:

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npm install effect

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or

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yarn add effect

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Also make sure you have TypeScript version 4.7 or higher installed:

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npm install typescript --save-dev

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To work with Effect-ts, you will need to configure tsconfig.json by enabling strict mode (strict: true) and, preferably, target: "ES2020" or higher.

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Quick start with code example

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Let's write a simple program that reads a user's name from the command line and outputs a greeting. This will show the basic concept of Effect-ts: describing an effect and executing it.

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import { Effect, Console } from "effect"\n\n// Describe the effect: ask for name\nconst askName: Effect.Effect<string> = Effect.sync(() => {\n  // In a real application, this would be keyboard input\n  return "Alexey"\n})\n\n// Describe the effect: output greeting\nconst greet = (name: string): Effect.Effect<void> =>\n  Console.log(`Hello, ${name}!`)\n\n// Combine effects\nconst program: Effect.Effect<void> = askName.pipe(\n  Effect.flatMap(greet)\n)\n\n// Run the program\nEffect.runSync(program)\n// Output: Hello, Alexey!

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In this example:

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• Effect.sync — creates an effect from a synchronous function.
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• Console.log — is an effect for console output.
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• pipe — allows sequentially applying functions to a value.
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• flatMap — used for sequential execution of effects.
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• Effect.runSync — executes the effect synchronously (only for synchronous effects).
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Main methods and functions of Effect-ts

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1. Effect.sync

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What it does: Creates an effect that wraps a synchronous function. The function is executed lazily (only when the effect is run).

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Parameters:

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• f: () => A — a synchronous function returning a value of type A.
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Example:

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import { Effect } from "effect"\n\nc