tiny-pattern-ts brings F#-style pattern matching to TypeScript. Patterns are ordinary objects whose matches method is a TypeScript type guard, so narrowing composes the way any other guard does. It is deliberately not a regex engine and not a macro: there is no transpiler and no DSL to learn, and the type-level contract is the feature — see development/library.md for the design decisions and Caveats for the limits.

Requirements

Node.js >= 26 (engines field; pinned via .node-version).
TypeScript >= 5.0 to consume the published declarations. The emitted .d.ts use const type parameters (TS 5.0) and keep their relative .ts specifiers; both resolve on TS >= 5.0 in node10 / node16 / nodenext / bundler.
The package is ESM-only (no CommonJS shim).

API

Yet to be implemented

Caveats

Only finite universes are supported. The factory must be given a finite union of literals; string, number and template literals are rejected. This is what lets the exhaustive overload be proven, so the runtime dispatch throw stays unreachable through the typed API.
A value and its stringification must not both be present. Object keys stringify, so a universe containing both a member and the string it stringifies to — 1 | "1", true | "true", null | "null" — is rejected at the factory. Either form alone is fine, and one value's string form may coexist with a different value's bare form ("true" | false).
symbol and bigint are not supported. A symbol brand is a compile-time phantom with nothing to match at runtime, and a bigint is not a valid property key; neither satisfies the matcher's universe constraint.
NaN and -0 cannot be matched specifically. They have no literal type, so both stay part of number.

Why open universes are rejected

An open universe — one carrying a broad member, as in type Units = "s" | "ms" | "min" | (string & {}) — is not a dispatch concern. If the values arrive from outside the program, parse them at the boundary down to a finite union and match the narrowed result; the openness never reaches the matcher. If the domain is genuinely extensible, the right shape is a runtime Map of handlers, where "no handler" is a lookup, not a pattern. Either way an open matcher would abandon the one guarantee this library exists to give — provable exhaustiveness — to automate what a switch and a default arm already cover. The type-level cost of supporting open universes is recorded in development/library.md.

License

Contributing

Contributions are documented in CONTRIBUTING.md; the reasons behind the project's decisions, rejected alternatives, and known issues live in development/. AI coding agents start at AGENTS.md.