launchdarkly-testing

Overview

LaunchDarkly's server-side SDK exposes a TestData data source that lets tests configure flag values without any network call - the canonical hermetic-test pattern. Per launchdarkly.com/docs/sdk, TestData replaces the production data source; everything else about the SDK (variation evaluation, targeting rules, default handling) runs the real code paths.

When to use

Unit / integration tests for code that calls client.variation() / client.boolVariation().
Tests asserting on targeting-rule behaviour (segment matching, percentage rollout).
Assignment-integrity tests per ab-test-validity-checklist.

Authoring

Install

npm install --save-dev launchdarkly-node-server-sdk
pip install launchdarkly-server-sdk

Initialize with TestData

import * as LaunchDarkly from 'launchdarkly-node-server-sdk';

const td = LaunchDarkly.TestData.dataSource();
const client = LaunchDarkly.init('sdk-test-key', {
  updateProcessor: td,
  sendEvents: false,         // No event uploads in tests
});

await client.waitForInitialization();

// Configure a flag's default + per-user variation
td.update(td.flag('show-new-ui').booleanFlag().on(true));

Variation evaluation

test('flag on returns true', async () => {
  td.update(td.flag('show-new-ui').booleanFlag().on(true));
  const user = { key: 'user-1' };
  const enabled = await client.variation('show-new-ui', user, false);
  expect(enabled).toBe(true);
});

test('flag off returns default', async () => {
  td.update(td.flag('show-new-ui').booleanFlag().on(false));
  const enabled = await client.variation('show-new-ui', { key: 'user-1' }, false);
  expect(enabled).toBe(false);
});

Targeting rules

test('only premium users get treatment', async () => {
  td.update(
    td.flag('premium-feature')
      .booleanFlag()
      .variationForUser('premium-user-1', true)
      .fallthroughVariation(false)
  );
  expect(await client.variation('premium-feature', { key: 'premium-user-1' }, false)).toBe(true);
  expect(await client.variation('premium-feature', { key: 'free-user-1' }, false)).toBe(false);
});

Percentage rollout (deterministic assignment)

test('rollout is deterministic per user', async () => {
  td.update(td.flag('half-rollout').booleanFlag().on(true).variations(true, false));
  // Bucketing is deterministic on user key
  const r1 = await client.variation('half-rollout', { key: 'user-1' }, false);
  const r2 = await client.variation('half-rollout', { key: 'user-1' }, false);
  expect(r1).toBe(r2);
});

File-based data source (alternative)

For fixture-shared tests:

const fileSource = LaunchDarkly.FileDataSource({
  paths: ['./tests/fixtures/ld-flags.json'],
});
const client = LaunchDarkly.init('sdk-test-key', {
  updateProcessor: fileSource,
  sendEvents: false,
});

The JSON file uses LaunchDarkly's flag-snapshot format. Useful for cross-language test fixtures.

Teardown

afterAll(async () => {
  await client.close();
});

Running

npm test

CI integration

jobs:
  ld-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v4
      - run: npm ci && npm test

No LAUNCHDARKLY_SDK_KEY needed in CI - TestData/file source replaces it.

Anti-patterns

Anti-pattern	Why it fails	Fix
Real SDK key in tests	Test traffic pollutes prod analytics	TestData / FileDataSource
`sendEvents: true` in tests	Event upload from CI	`sendEvents: false`
Multiple init per test	Slow init; leaks	One global client; TestData.update per test
Asserting exact variation result without `td.update`	Race vs SDK state	Always `update` first
Forgetting `await client.waitForInitialization()`	Race; variation returns default	Always wait
Sharing `td` across test files	Cross-test pollution	Per-file or per-test TestData
No teardown `client.close()`	Network handles leak	Always close in afterAll

Limitations

TestData is server-side-SDK-specific. Client-side SDKs have their own offline modes.
Doesn't validate LaunchDarkly's UI-side rollout math. Platform-side bucketing is separately tested.
No experiment-results validation. That's experiment-results-interpreter.
FileDataSource schema is LaunchDarkly-internal. Refer to LD docs for the format.

References

LaunchDarkly SDK docs: launchdarkly.com/docs/sdk.
TestData data source guide: launchdarkly.com/docs/sdk/features/test-data-sources.
File-based data source: launchdarkly.com/docs/sdk/features/flags-from-files.
Deciding WHICH flag combinations to cover is a separate step this skill does not do: when code branches on several flags, load openfeature-sdk-testing (its flag test-matrix reference) for the matrix, then come back here to pin each flag's value per test.
Sibling SDK umbrella: openfeature-sdk-testing (OpenFeature + Unleash / Flagsmith / GrowthBook references).
Cross-plugin: feature-flag-test-harness, ab-test-validity-checklist.