@youneed/test-plugin-feature-flags

Deterministic feature flags in @youneed/test. Flags are murder in tests: one case flips a flag on, the next inherits it, and now your suite passes or fails by order. This package gives every test a fresh FeatureFlags engine (overrides wiped between cases) plus a scoped withFlags(...) for forcing a value for exactly one block.

import { Test, TestApplication, expect } from "@youneed/test";
import { flagsFixture, withFlags, expectFlag } from "@youneed/test-plugin-feature-flags";
import type { FeatureFlags } from "@youneed/feature-flags";

const Flags = flagsFixture([
  { key: "new-checkout", defaultValue: false },
  { key: "theme", defaultValue: "light", variants: { light: "light", dark: "dark" } },
]);

class Checkout extends Test() {
  @Test.use(Flags) flags!: FeatureFlags; // …or decorator-free: flags = Flags.get();

  @Test.it("off by default") off() {
    expect(this.flags.isEnabled("new-checkout")).toBeFalsy();
  }

  @Test.it("forced on for THIS test only") on() {
    this.flags.override("new-checkout", true);
    expectFlag(this.flags, "new-checkout");
    // the next test gets a fresh engine — no leak.
  }

  @Test.it("scoped override restores after the block") scoped() {
    withFlags(this.flags, { "new-checkout": true }, () => {
      expectFlag(this.flags, "new-checkout"); // enabled inside
    });
    expect(this.flags.isEnabled("new-checkout")).toBeFalsy(); // restored
  }
}

TestApplication().addTests(Checkout).run();

flagsFixture is scoped "test" by default, so a new engine is constructed for every case — any override(...) a test applies is gone for the next one, no manual reset needed. Consume it with @Test.use(Fix) or decorator-free via Fix.get(), exactly like any other @youneed/test fixture.

| API | meaning | | --- | --- | | flagsFixture(defs?, opts?) | a @youneed/test fixture providing a fresh FeatureFlags per test; resets overrides between tests | | withFlags(flags, overrides, fn) | run fn with forced overrides, restoring prior state after (sync + async, even on throw) | | expectFlag(flags, key, ctx?) | throw an AssertionError unless the flag evaluates truthy | | expectFlagDisabled(flags, key, ctx?) | throw unless the flag evaluates falsy |

| flagsFixture option | default | meaning | | --- | --- | --- | | name | "feature-flags" | fixture display name | | scope | "test" | fixture scope; keep "test" for per-case isolation ("suite"/"run" share one engine, teardown clears overrides when the scope ends) |