JavaScript browser SDK

@flagfix/browser is the client SDK for SPAs and browser bundles. It evaluates flags with evaluate-public, streams updates over SSE (unless disabled), and queues events to /api/v1/ingest. Set apiHost to https://api.flagfix.io, or to the API host your company uses.

import { createFlagFix } from "@flagfix/browser";

const client = createFlagFix({
  clientKey: process.env.NEXT_PUBLIC_FLAGFIX_CLIENT_KEY!,
  apiHost: process.env.NEXT_PUBLIC_FLAGFIX_API_HOST ?? "https://api.flagfix.io",
  storage: { mode: "persistent" },
});
await client.start();

Method	Returns
`start()`	`Promise<StartResult>` with `ready`, `storageMode`, `persistenceLevel`, optional `reason`
`identify(userId, traits?)`	`TrackReceipt`. Drops the previous flag snapshot
`setTraits` / `setTrait` / `reset`	`void`
`track(name, properties?)`	`TrackReceipt`
`trackDurable(name, properties?)`	`Promise<TrackDurableResult>`
`trackExposure(key, details?)`	receipt, or `{ queued: false, reason: "DUPLICATE_EXPOSURE" }`
`hasFeature(key, { fallback? })`	`boolean`. Does not emit an exposure
`getValue(key, { fallback? })`	the flag value or the fallback
`getAllFlags()`	`Record<string, FlagResult>`
`getFlagMeta()` / `getFlagState()`	version, source, etag
`refreshFlags()`	`Promise<FlagState \| null>`
`subscribeFlags(listener)`	unsubscribe function
`flush(timeoutMs?)` / `shutdown(timeoutMs?)`	`Promise<FlushResult>`
`resumeExport()` / `isExportSuspended()`	after HTTP 401 or 403
`getIdentity()`	`IdentityState`
`diagnostics()`	`Promise<DiagnosticsSnapshot>`

Evaluation mode is "public" (evaluate-public). SSE is on unless flags overrides it. lifecycle: false disables the document listener in tests. An empty clientKey throws.