React SDK - API reference

`<NexusProvider>`

<NexusProvider
  config={NexusConfig}
  loadingFallback={ReactNode}
  onError={(error: Error) => ReactNode}
>
  {children}
</NexusProvider>

Creates the client, holds children back until the first sync completes, and closes the client on unmount. See Configuration for config. Without onError, an initialisation failure is re-thrown so a React error boundary can handle it.

Value hooks

Every value hook is built on useSyncExternalStore, so a concurrent render cannot show one flag’s old value beside another’s new one.

`useConfig<T>(key): T | undefined`

The resolved value of a config key, or undefined when absent. The type parameter is a cast.

`useFlag(key, defaultValue = false): boolean`

The flag’s is_active state, or defaultValue when the key is absent.

`useNexusKind(): string`

The service kind the server reported: "backend", "frontend", or "" when the server omitted it.

`useNexusSyncedAt(): number`

The last successful sync, as epoch milliseconds. A number rather than a Date, because a hook must return a stable value for an unchanged snapshot and a fresh Date is a new reference every read.

const syncedAt = useNexusSyncedAt();
return <small>Updated {new Date(syncedAt).toLocaleTimeString()}</small>;

`useNexusBillingOverdue(): boolean`

true while the server answers HTTP 402. Reads keep working from the cache and the refresh loop slows rather than stopping, so this clears on its own once the invoice is settled.

`useNexusStreamStatus(): { connected, state }`

The live stream’s status. state is one of idle, connecting, connected, reconnecting, polling. Updates on every transition, not only when a sync replaces the cache.

Actions

`useNexus(): NexusClient`

The client itself. Throws when called outside a <NexusProvider>.

`useNexusSync(): () => Promise<void>`

Refreshes the cache. Concurrent calls share one request.

`useEvaluateAB(): (keys, userId, attributes?: Record<string, string>) => Promise<Record<string, boolean>>`

Evaluates flags through the AB Testing endpoint. Always a live call. In-flight requests are aborted when the component unmounts. Rejects with NexusAbAddonNotAvailableError when the add-on is not active.

`NexusClient`

Usually reached through the hooks. Direct use is for non-React code paths.

const client = await NexusClient.create(config);

Member	Returns
`getConfig(key)` / `getConfigAs<T>(key)`	`unknown` / `T \| undefined`
`getFlag(key, defaultValue?)`	`boolean`
`hasFlag(key)`	`boolean` - whether the service defines the flag at all, which `getFlag` cannot express
`getFlagKeys()`	`string[]` - every flag key in the current snapshot
`sync()`	`Promise<void>`, coalesced
`evaluateAB(keys, userId, attributes?, signal?)`	`Promise<Record<string, boolean>>`; `attributes` is `Record<string, string>` - the endpoint compares cohort values as strings
`subscribe(listener)`	an unsubscribe function; fires on any observable change
`connectStream()` / `disconnectStream()`	the live stream
`close()`	final; releases the stream, every timer and every listener
`detectedKeyType`	`"pk"` or `"sk"`
`kind`	the service kind
`syncedAt`	`Date`
`billingOverdue`	`boolean`
`quarantinedUntil`	`Date \| null`
`streamConnected` / `streamState`	the live stream’s status
`isClosed`	`boolean`

Types and constants

NexusConfig, NexusLogger, NexusLogFields, NexusStreamObserver, NexusStreamState, NexusChangeListener, NexusProviderProps, NexusProviderStatus, NexusStreamStatus, PublicApiKey. assertPublicKey(key) narrows a string to PublicApiKey, throwing if it is not a public key - the same check the client applies to apiKey. NOOP_LOGGER, NOOP_STREAM_OBSERVER, FALLBACK_REASON_MAX_ERRORS, FALLBACK_REASON_RATE_LIMITED, FALLBACK_REASON_TRANSPORT.

Errors

See Error handling for the full hierarchy and which errors you will actually encounter.

Removed in v0.16.0

Removed	Why
`getSecret()`, `NexusPublicKeyError`, `NexusSecretNotFoundError`	A browser client holds a public key, and the backend never sends a secret to one, so the method could only ever throw. Secrets belong in a backend SDK.

Removed in v0.11.0

Removed	Why
`useSecret()`	A browser client holds a public key, so it could only ever return `undefined`. Secrets belong in a backend SDK.

Overview

Backend SDKs

Browser SDKs

OpenFeature Providers

React SDK - API reference

`<NexusProvider>`

Value hooks

`useConfig<T>(key): T | undefined`

`useFlag(key, defaultValue = false): boolean`

`useNexusKind(): string`

`useNexusSyncedAt(): number`

`useNexusBillingOverdue(): boolean`

`useNexusStreamStatus(): { connected, state }`

Actions

`useNexus(): NexusClient`

`useNexusSync(): () => Promise<void>`

`useEvaluateAB(): (keys, userId, attributes?: Record<string, string>) => Promise<Record<string, boolean>>`

`NexusClient`

Types and constants

Errors

Removed in v0.16.0

Removed in v0.11.0

​<NexusProvider>

​Value hooks

​useConfig<T>(key): T | undefined

​useFlag(key, defaultValue = false): boolean

​useNexusKind(): string

​useNexusSyncedAt(): number

​useNexusBillingOverdue(): boolean

​useNexusStreamStatus(): { connected, state }

​Actions

​useNexus(): NexusClient

​useNexusSync(): () => Promise<void>

​useEvaluateAB(): (keys, userId, attributes?: Record<string, string>) => Promise<Record<string, boolean>>

​NexusClient

​Types and constants

​Errors

​Removed in v0.16.0

​Removed in v0.11.0

`<NexusProvider>`

Value hooks

`useConfig<T>(key): T | undefined`

`useFlag(key, defaultValue = false): boolean`

`useNexusKind(): string`

`useNexusSyncedAt(): number`

`useNexusBillingOverdue(): boolean`

`useNexusStreamStatus(): { connected, state }`

Actions

`useNexus(): NexusClient`

`useNexusSync(): () => Promise<void>`

`useEvaluateAB(): (keys, userId, attributes?: Record<string, string>) => Promise<Record<string, boolean>>`

`NexusClient`

Types and constants

Errors

Removed in v0.16.0

Removed in v0.11.0