mitt

mitt is an extremely small functional event emitter/pubsub library, weighing under 200 bytes gzipped, that gives JavaScript applications a familiar on/off/emit API modeled after Node’s EventEmitter. Unlike class-based emitters, mitt’s methods are purely functional and don’t rely on this, and it adds a wildcard '*' event type that listens to every emitted event.

Written in TypeScript, mitt provides strong generic typing for event maps when strict mode is enabled in a consuming project’s tsconfig.json, letting handlers infer their event payload types automatically. It has no dependencies, supports IE9+, and works in any JavaScript runtime, making it a common choice for decoupled component communication in frameworks like Vue and Preact as well as vanilla JavaScript applications.

What You Get

• A functional on/off/emit event emitter API modeled after Node’s EventEmitter, with no reliance on this
• A wildcard '*' event type that listens to every emitted event, receiving both the event type and payload
• Strong TypeScript generics — define an Events type map and get inferred, type-checked handler and payload types
• Direct access to the underlying all Map of event names to handler sets, for bulk operations like clearing all listeners
• Zero dependencies and IE9+ support, usable in any JavaScript runtime (browser, Node.js, or bundled via ESM/CJS/UMD)

Common Use Cases

• Decoupled communication between components in frameworks without a built-in event bus (e.g. Preact, Vue 3 which dropped its own $on/$off)
• Implementing a lightweight application-wide event bus for cross-cutting concerns like analytics or notifications
• Building small state or plugin systems that need publish/subscribe semantics without pulling in a full framework
• Prototyping or teaching event-driven patterns where a minimal, readable emitter implementation is preferred over Node’s built-in EventEmitter in browser code

Under The Hood

Architecture - mitt is implemented as a single ~120-line TypeScript file (src/index.ts) exporting one factory function that creates an emitter instance backed by a Map<EventType, Handler[]>; on, off, and emit are plain functions closing over that map rather than class methods, which is what allows the entire implementation to be so small and to avoid this-binding footguns. Tech Stack - Pure TypeScript with no runtime dependencies, compiled to ESM, CommonJS, and UMD builds for universal consumption; the UMD build is distributed via unpkg for direct <script> tag usage. Code Quality - The repository includes a dedicated index_test.ts covering core emitter behavior and a test-types-compilation.ts file specifically verifying the TypeScript generic inference works as documented — an unusually thorough test investment for a library this small, reflecting how central correct typing is to its value proposition. API Design - The three-method surface (on, off, emit) plus the wildcard '*' convention is deliberately tiny and immediately familiar to anyone who has used Node’s EventEmitter, while the TypeScript generics let consumers get compile-time safety on event names and payload shapes with minimal ceremony — a rare combination of minimalism and type safety.