SDKs
Unit testing
Mock the network with fetchImpl. Keep tests offline and deterministic.
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Goals
- Assert flag reads and event queueing without a live API
- Keep tests fast and hermetic (CI-friendly)
- Avoid leaking real SDK keys into fixtures
Browser / Node pattern
Pass a fake fetchImpl that returns the same JSON shapes as the runtime OpenAPI (EvaluateAllResponse, ingest 202, etc.).
import { createFlagFix } from "@flagfix/node";
import { describe, it, expect } from "vitest";
describe("checkout flag", () => {
it("reads enabled from evaluate-all", async () => {
const client = createFlagFix({
serverKey: "test-server-key",
apiHost: "http://flagfix.test",
lifecycle: false, // browser only: disable document listeners
fetchImpl: async (input) => {
const url = String(input);
if (url.includes("/evaluate-all") || url.includes("/evaluate-public")) {
return new Response(
JSON.stringify({
flags: [
{
key: "new_checkout",
enabled: true,
value: null,
configurationVersion: 1,
},
],
matchedSegments: [],
configurationVersion: 1,
}),
{ status: 200, headers: { "content-type": "application/json" } },
);
}
if (url.includes("/ingest")) {
return new Response(JSON.stringify({ acceptedIds: ["1"] }), {
status: 202,
headers: { "content-type": "application/json" },
});
}
return new Response("not mocked", { status: 500 });
},
});
await client.start();
expect(client.hasFeature("new_checkout")).toBe(true);
});
});React
- Wrap components in
FlagFixProviderwith a pre-builtclient(or mocked options +fetchImpl) - Set
lifecycle: falseon browser options inside tests - Prefer
serverFlagStatefixtures to assert first paint without waiting on network
import { FlagFixProvider, useHasFeature } from "@flagfix/react";
import { render, screen } from "@testing-library/react";
function Banner() {
const on = useHasFeature("new_checkout");
return <div>{on ? "on" : "off"}</div>;
}
it("renders SSR snapshot", () => {
render(
<FlagFixProvider
options={{ clientKey: "test", apiHost: "http://flagfix.test", lifecycle: false }}
serverFlagState={{
flags: { new_checkout: { key: "new_checkout", enabled: true, value: null } },
configurationVersion: 1,
source: "ssr",
}}
>
<Banner />
</FlagFixProvider>,
);
expect(screen.getByText("on")).toBeInTheDocument();
});What not to do
- Do not point unit tests at production or shared staging
- Do not assert on wall-clock flush timing without controlling the clock
- Do not import
@flagfix/sdk-core. Test through@flagfix/browser,@flagfix/node, or@flagfix/react
Related: How to test flag evaluation
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