OpenFeature

OpenFeature is a vendor-neutral API for feature flags. @faststats/openfeature provides FastStats providers for its server and web SDKs, so you can read FastStats flags through the OpenFeature client you already use.

Already using @faststats/web or @faststats/react in the browser? The built-in SDK shares the visitor and session with analytics and records exposures only for flags you read.

Authentication

Both providers take a token: the project’s public token, or a FastStats API key scoped to the project with the featureFlagEvaluation:evaluate permission. Only use an API key on the server.

Server


npm install @openfeature/server-sdk @faststats/openfeature


import { FastStatsProvider } from "@faststats/openfeature/server";
import { OpenFeature } from "@openfeature/server-sdk";

await OpenFeature.setProviderAndWait(
	new FastStatsProvider({ token: process.env.FASTSTATS_FLAGS_TOKEN! }),
);

const client = OpenFeature.getClient();

const newCheckout = await client.getBooleanValue("new-checkout", false, {
	targetingKey: visitorId,
	userId: user?.id,
	userAgent: request.headers.get("user-agent") ?? undefined,
	plan: "pro",
});

Each evaluation is one request for that one flag, and is recorded as an exposure.

On the server, the flags service sees your server rather than the visitor. Pass userAgent so rules can match browser, os, and device, and set country yourself if your rules use it.

Web


npm install @openfeature/web-sdk @faststats/openfeature


import { FastStatsProvider } from "@faststats/openfeature/web";
import { OpenFeature } from "@openfeature/web-sdk";

await OpenFeature.setContext({ targetingKey: visitorId, plan: "pro" });
await OpenFeature.setProviderAndWait(
	new FastStatsProvider({
		token: "your_project_token",
		flags: ["new-checkout", "max-uploads"],
	}),
);

const client = OpenFeature.getClient();
const newCheckout = client.getBooleanValue("new-checkout", false);

Flags load once, and again whenever the context changes, then resolve from memory. Browser, OS, device, and country come from the visitor’s own request.

The web provider records every flag it loads as an exposure, whether or not the app reads it. List the keys your app uses in flags, or every flag in the project is loaded and counted.

Evaluation Context

Context field	Sent as
`targetingKey`	`identifier`: a stable device or install ID. Required unless `userId` is set
`userId`	Your ID for a signed-in user. Rollouts bucket by it, so they follow the user
`sessionId`	Links evaluations to a session in analytics
`userAgent`	Server only: the visitor’s `User-Agent`, for `browser`, `os`, and `device` rules
anything else	An attribute targeting rules can match on

Defaults and Errors

A flag resolves to the default passed at the call site when it does not exist, is archived, has a different type, or the service cannot be reached within timeoutMs. The evaluation details carry the matching OpenFeature error code.


const details = await client.getBooleanDetails("new-checkout", false, context);

details.variant; // "on"
details.reason; // OpenFeature reason, such as "TARGETING_MATCH"
details.flagMetadata.reason; // FastStats reason, such as "rule_match"

FastStats reason	OpenFeature reason
`rule_match`	`TARGETING_MATCH`
`target_match`	`TARGETING_MATCH`
`fallthrough`	`DEFAULT`
`disabled`	`DISABLED`
`prerequisite_failed`	`DISABLED`

Options

Option	Type	Default	Description
`token`	`string`	required	Project token or API key
`endpoint`	`string`	`https://flags.faststats.dev`	Base URL of the flags service
`timeoutMs`	`number`	`3000`	Aborts a slower request
`flags`	`string[]`	all flags	Web only: the flag keys to load
`fetch`	`typeof fetch`	global `fetch`	A `fetch` implementation to use instead

OpenFeature

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ServerCopy Anchor Link

WebCopy Anchor Link

Evaluation ContextCopy Anchor Link

Defaults and ErrorsCopy Anchor Link

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On this page

Authentication

Server

Web

Evaluation Context

Defaults and Errors

Options