Integrate LaunchDarkly feature flags in a Node.js service with server-side SDK evaluation

Steps

1. Install the SDK: `npm install @launchdarkly/node-server-sdk`
2. Initialize the client at application startup with your SDK key: `const client = ld.init(YOUR_SDK_KEY)`; call `await client.waitForInitialization()` before evaluating flags to ensure the flag ruleset is loaded
3. Evaluate a flag by calling `client.variation('your-flag-key', userContext, defaultValue)` where `userContext` contains a `key` (unique user or entity identifier) and any targeting attributes
4. Use `client.variationDetail()` if you need the evaluation reason (targeting rule matched, fallthrough, off, etc.) for logging or debugging
5. Stream flag changes automatically — the server-side SDK maintains a persistent connection to LaunchDarkly's streaming endpoint and updates flags in memory without requiring restart
6. Shut down cleanly with `client.close()` on process exit to flush any pending analytics events