@formancy/core

The headless form engine: one build that runs unchanged in a browser and on a server, so client and server can never disagree about what a form means.

No framework imports, no DOM, no Node globals. The absence of @types/node is deliberate — it makes the typecheck enforce the constraint instead of a reviewer having to.

Two load-bearing decisions

Snapshots are identity-stable. getFieldSnapshot(path) returns the same frozen object until that field's value, visibility, requiredness, touched state or errors actually change. That is what lets React's useSyncExternalStore and Angular's zoneless change detection work with no memoisation on the consumer's side: a keystroke on a five-thousand-field form re-renders exactly the fields it touched. (Measured: ~0.4 ms of engine work per keystroke at that size.)

Failure direction is chosen per concern. Compilation failures — an unparseable expression, a ghost field reference, a dependency cycle — are refused when the engine is built, so a broken form never reaches a person. Evaluation failures during typing are normal, and there metadata fails open (a broken visibility rule shows the field, rather than silently swallowing what someone typed) while validation fails closed (a rule that cannot evaluate cannot vouch for the value).

Use

import { createFormEngine } from '@formancy/core'

const engine = createFormEngine({
  schema,
  capabilities: { now: Date.now, today: () => '2026-09-20', random: Math.random },
})

engine.subscribeField(['email'], () => render())
engine.setValue(['email'], '[email protected]')
const { value, errors, props, ids } = engine.getFieldSnapshot(['email'])
const outcome = engine.submit()

The engine also mints accessibility ids and composes aria-describedby centrally, so every renderer emits identical wiring rather than three teams each getting it nearly right.

Docs: apps/docs.