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Node.js (Server) — SDK detail

• Official docs: Node.js SDK reference (server-side)
• API reference: SDK API docs
• Recipe (detect / install): SDK Recipes (Node.js Server)

Includes: Patterns below follow the Get started, Initialize the client, Evaluate a context, and Promises and async sections of the Node.js server-side SDK reference. Use one initialization strategy (waitForInitialization or the ready event—not both at once unless you know why).

Singleton client

LDClient must be a singleton per LaunchDarkly environment—do not create a new client per request. Use your real SDK key from env vars, not a literal in source.

import { init } from '@launchdarkly/node-server-sdk';

const client = init(process.env.LAUNCHDARKLY_SDK_KEY);

Wait for initialization (startup)

Run this once during process startup—for example before your HTTP server accepts connections, inside whatever async bootstrap your framework provides. The public docs use { timeout: 10 } (seconds); adjust as needed.

Promise style (from Promises and async):

client
  .waitForInitialization({ timeout: 5 })
  .then(() => {
    // Initialization complete — safe to evaluate flags and/or start serving traffic.
  })
  .catch((err) => {
    // Timeout or initialization failed
  });

Async/await (same topic—must live inside an async function your app already uses for startup):

try {
  await client.waitForInitialization({ timeout: 5 });
  // Initialization complete
} catch (err) {
  // Timeout or initialization failed
}

Alternative: the docs also document a ready event and callback-style client.variation(..., (err, value) => { ... }) under Evaluate a context—use that form if it fits your codebase better.

Evaluate a flag (when handling work)

The docs state that in production you should invoke variation as needed (not only once at import). Use await inside an async route handler, service method, job, etc.

const context = {
  kind: 'user',
  key: 'example-user-key',
  name: 'Example User',
};

const showFeature = await client.variation('example-flag-key', context, false);

For a boolean flag you may use boolVariation if you prefer; the official getting-started examples use variation with a boolean default.