Split.io (Harness FME) SDK testing

The Split.io SDK (now Harness Feature Management & Experimentation, FME) keeps the surface SplitFactory, getTreatment, getTreatmentWithConfig, and the event system. Both the JavaScript (browser) and Node.js (server-side) SDKs support a localhost/offline mode that eliminates all network calls during tests, making feature-flag evaluation fully hermetic. This skill covers both; all SDK behavior cited below is drawn from the FME SDK docs (see References).

Differentiation from the sibling vendor references: statsig.md uses localMode: true (gate/config primitives); optimizely.md uses a JSON datafile; amplitude.md uses a local-eval JSON fixture. Split.io is distinct: Split.io's offline mechanism uses authorizationKey: 'localhost' paired with an in-memory features map (JS SDK) or a YAML/text fixture file (Node.js SDK), and its evaluation API is getTreatment / getTreatmentWithConfig rather than decide, variant, or checkGate.

When to use

• Tests for code that calls client.getTreatment() or client.getTreatmentWithConfig() against a Split.io-instrumented surface.
• Verifying that impression listeners fire correctly per SDK evaluation.
• Assignment-integrity tests per ab-test-validity-checklist Step 3.
• CI pipelines where network access to Split.io / Harness is unavailable or undesirable.

How to use

Install

npm install --save-dev @splitsoftware/splitio   # Node.js + browser

Localhost/offline mode - JavaScript (browser) SDK

Set authorizationKey to 'localhost' and supply a features map:

import { SplitFactory } from '@splitsoftware/splitio';

const factory = SplitFactory({
  core: {
    authorizationKey: 'localhost',
    key: 'test-user-1',
  },
  features: {
    'checkout_redesign':  'on',
    'dark_mode':          'off',
    'pricing_experiment': { treatment: 'v2', config: '{"price":9}' },
  },
  scheduler: {
    offlineRefreshRate: 15,   // seconds between simulated polls
  },
});
const client = factory.client();

Any flag absent from the features map returns the 'control' treatment automatically - no extra setup needed for flags the test does not care about.

Localhost/offline mode - Node.js (server-side) SDK

The server-side SDK reads offline fixtures from a file path. Use a YAML fixture (supported since SDK v10.7.0) for per-key targeting:

# tests/fixtures/split-flags.yml
- checkout_redesign:
    treatment: "on"
    keys: "test-user-1"
    config: "{}"
- checkout_redesign:
    treatment: "off"
- dark_mode:
    treatment: "off"

import path from 'path';
import { SplitFactory } from '@splitsoftware/splitio';

const factory = SplitFactory({
  core: { authorizationKey: 'localhost' },
  features: path.join(__dirname, 'fixtures/split-flags.yml'),
  scheduler: { offlineRefreshRate: 15 },
});
const client = factory.client('test-user-1');

The plain-text format (two whitespace-separated columns) is also supported for simpler cases where per-key targeting is not needed.

SDK_READY event and whenReady()

The SDK emits client.Event.SDK_READY when its data is loaded. Always wait for this event before evaluating treatments to avoid receiving 'control' prematurely:

// Event-listener style
client.on(client.Event.SDK_READY, () => {
  const treatment = client.getTreatment('checkout_redesign');
  expect(treatment).toBe('on');
});

Or use the promise-based equivalent (cleaner in async test bodies):

// Promise style
beforeAll(async () => {
  await client.whenReady();
});

Additional events:

• SDK_READY_TIMED_OUT: timeout before data loaded; SDK may still become ready later
• SDK_UPDATE: rollout plan changed (useful in localhost mode to test dynamic flag flips)

getTreatment and getTreatmentWithConfig

getTreatment returns a treatment string; getTreatmentWithConfig returns { treatment: string, config: string | null }:

test('user in treatment arm sees new pricing', async () => {
  await client.whenReady();

  const treatment = client.getTreatment('test-user-1', 'pricing_experiment');
  expect(treatment).toBe('v2');

  const result = client.getTreatmentWithConfig(
    'test-user-1',
    'pricing_experiment'
  );
  expect(result.treatment).toBe('v2');
  expect(JSON.parse(result.config!)).toEqual({ price: 9 });
});

test('unknown flag returns control', async () => {
  await client.whenReady();
  const treatment = client.getTreatment('test-user-1', 'nonexistent_flag');
  expect(treatment).toBe('control');
});

Note: the Node.js SDK's getTreatment takes (key, flagName, attributes?, evaluationOptions?). The JavaScript client SDK's getTreatment takes (flagName, attributes?) because the key is bound at factory.client(key) construction time.

Impression listener verification

Attach an impressionListener to SplitFactory options. The logImpression callback receives an object containing impression (feature flag, key, treatment, label), attributes, ip, hostname, and sdkLanguageVersion:

const impressions: any[] = [];

const factory = SplitFactory({
  core: { authorizationKey: 'localhost' },
  features: path.join(__dirname, 'fixtures/split-flags.yml'),
  impressionListener: {
    logImpression(data) {
      impressions.push(data);
    },
  },
});
const client = factory.client('test-user-1');

test('impression fires on getTreatment', async () => {
  impressions.length = 0;
  await client.whenReady();

  client.getTreatment('test-user-1', 'checkout_redesign');

  expect(impressions).toHaveLength(1);
  expect(impressions[0].impression.treatment).toBe('on');
});

Controlling impression mode

sync.impressionsMode has three values:

• 'OPTIMIZED' (default): only unique impressions queued; reduces traffic; suitable for production and most test scenarios.
• 'DEBUG': all impressions queued and sent; use when validating that every evaluation generates a record.
• 'NONE': no impressions tracked; use for flag-only (non-experiment) use cases in CI where impression noise is unwanted.

const factory = SplitFactory({
  core: { authorizationKey: 'localhost' },
  features: path.join(__dirname, 'fixtures/split-flags.yml'),
  sync: { impressionsMode: 'DEBUG' },
});

Assignment integrity tests

Per ab-test-validity-checklist Step 3, verify that the same key always receives the same treatment and that different keys can receive different treatments:

test('same key always gets same treatment (determinism)', async () => {
  await client.whenReady();
  const t1 = client.getTreatment('test-user-1', 'checkout_redesign');
  const t2 = client.getTreatment('test-user-1', 'checkout_redesign');
  expect(t1).toBe(t2);
});

test('treatments match the features map (hermetic)', async () => {
  await client.whenReady();
  expect(client.getTreatment('test-user-1', 'checkout_redesign')).toBe('on');
  // Different key receives the default treatment (no per-key override)
  expect(client.getTreatment('test-user-2', 'checkout_redesign')).toBe('off');
});

Teardown

Call client.destroy() (returns a promise) after tests to release internal resources. Skipping it leaks event-listener handles and impression flush timers:

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

After destroy() is called, subsequent getTreatment calls return 'control' and factory operations require re-instantiation.

CI integration

jobs:
  split-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v4
        with:
          node-version: '20'
      - run: npm ci
      - run: npm test
        # No SPLIT_API_KEY needed: localhost mode is fully offline

The YAML fixture is committed alongside the test code. No SPLIT_API_KEY secret is required in CI when using localhost mode.

Example

A full end-to-end Node.js/Jest test for a checkout feature guarded by two flags is in split-io-example.md.

Anti-patterns

Limitations

• No arm-pinning override API. Unlike Statsig's overrideGate or Optimizely's setForcedDecision, the Split.io SDK has no per-user override call. Pin treatments by adding a per-key YAML entry in the fixture file or by supplying the exact key that maps to the desired treatment.
• Localhost mode is in-process only. It does not work across multiple processes (e.g., a forked worker) unless each process initializes its own factory with the same fixture.
• YAML per-key targeting requires Node.js SDK v10.7.0+. Earlier versions only support the plain-text two-column format with uniform treatment per flag.
• Dynamic feature-map mutations (JS SDK only). Mutating the features object at runtime triggers SDK_UPDATE and simulates a rollout change. The Node.js file-based fixture does not support runtime mutation without reloading.
• Does not validate Split.io's server-side statistical analysis. Platform analysis is the vendor's responsibility; this skill tests your code's interaction with the SDK only.

References

• Node.js SDK docs (Harness FME): developer.harness.io/docs/feature-management-experimentation/sdks-and-infrastructure/server-side-sdks/nodejs-sdk
• JavaScript SDK docs (Harness FME): developer.harness.io/docs/feature-management-experimentation/sdks-and-infrastructure/client-side-sdks/javascript-sdk
• Companion: ab-test-validity-checklist; result-trust references (peeking, guardrails) live in experiment-results-interpreter
• Sibling vendor references: statsig.md, optimizely.md, amplitude.md, vwo.md