bimorph

Bidirectional data mapping for TypeScript. Define a mapping once and get both directions — decode (wire → domain) and encode (domain → wire) — with the reverses that don't invert cleanly surfaced in the type instead of failing silently.

import { Enum } from 'bimorph';

const Status = Enum([
    [1, 'active'],
    [0, 'inactive'],
]);

Status.decode(1); // 'active'   — wire → domain
Status.encode('active'); // 1   — domain → wire

One declaration, both directions — no hand-written reverse lookup drifting out of sync.

The problem it kills

The happy path is easy; a lookup object already does that. The pain is the reverse:

const STATUSES = { 0: 'BAD', 1: 'OK' };
const label = STATUSES[0]; // easy
const value = Object.entries(STATUSES).find(([, v]) => v === 'OK')?.[0]; // ugh — and it drifts

And most real mappings don't invert cleanly: duplicate values, lossy transforms, missing keys, read-only fields, reverses that need runtime context the value doesn't carry. Most tools ignore that or hide it. bimorph's thesis: surface non-invertibility in the type, and give explicit escape hatches — instead of pretending every mapping is a clean isomorphism.

That shows up as:

• Fidelity tiers — iso (exact round-trip), lossy (no round-trip promised), partial (the inverse can fail). A partial codec doesn't even offer a throwing decode; the type forces you through safeDecode.
• Doors, not flags — decode throws with a path, safeDecode returns a Result, decodeOr falls back, validate accumulates every error. The behavior is visible at the call site, never a hidden config flag.
• Aliases — several wire values decode to one domain value, but encode only ever emits the canonical one (narrowed in the type), so a migration can't leak a legacy spelling.
• Context — a codec that needs a region or locale says so in its type; you can't silently forget to pass it.

Install

npm install bimorph

Early release — expect breaking changes before 1.0. The API has just stabilized, but the package currently ships TypeScript source (src/index.ts): it works in a TypeScript project with a bundler, and a compiled dist/ (.js + .d.ts) for plain Node/JS is on the way. Pin the version.

Documentation

The full documentation site — Fumadocs, with every ts twoslash example type-checked against the real types at build time — lives in apps/docs/. Run it with npm run docs and open http://localhost:3411.

Design record:

• docs/DESIGN.md — principles, the codec contract, and the API surface.
• docs/SCENARIOS.md — ~28 real-world mappings the design was tested against.
• docs/DOGFOOD.md — the design written against those scenarios, and the gaps it exposed.

Development

npm run typecheck     # tsc --noEmit over src/
npm run check:runtime # runtime-behaviour regression gate
npm run docs          # the documentation site at http://localhost:3411

License

MIT © 2026 Oleksandr Zhuravlov