GitLab feature flags: what you get, the limits, and when to outgrow them

// npm install unleash-client
const { initialize } = require('unleash-client');

// All three values come from Deploy > Feature flags > Configure
const unleash = initialize({
  // Project-scoped Unleash API. The SDK appends /client/features itself.
  url: 'https://gitlab.com/api/v4/feature_flags/unleash/42',

  // GitLab calls this the "Instance ID". It is the auth token.
  instanceId: process.env.GITLAB_UNLEASH_INSTANCE_ID,

  // GOTCHA: "Application name" is NOT your app name.
  // It must match the ENVIRONMENT spec on the flag: production, staging...
  appName: process.env.GITLAB_ENVIRONMENT ?? 'production',

  refreshInterval: 15000, // ms. Lower = more load on GitLab's API.
});

unleash.on('ready', () => {
  // Percent of users and user-ID strategies REQUIRE a user id.
  // Without it, the flag evaluates false for that visitor.
  const on = unleash.isEnabled('checkout.new-flow', {
    userId: String(currentUser.id),
  });

  render(on ? newCheckout() : legacyCheckout());
});

// Fail closed, not loudly: keep the old path if GitLab is unreachable.
unleash.on('error', (err) => log.warn('unleash', err));