Documentation for dd-trace with feature flagging support

content/en/feature_flags/setup/nodejs.md

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+---
+title: Node.js Feature Flags Support
+description: Set up Datadog Feature Flags for Node.js applications.
+further_reading:
+- link: "/feature_flags/setup/"
+  tag: "Documentation"
+  text: "Feature Flags Setup"
+- link: "/apm/"
+  tag: "Documentation"
+  text: "APM Setup"
+---
+
+## Installing and initializing
+
+Feature Flagging is provided by APM. To integrate APM into your application with feature flagging support, install `dd-trace` and enable remote configuration with the `flaggingProvider` option as shown below. See documentation on `dd-trace` integration for detailed APM installation instructions.
+
+```shell
+npm install dd-trace @openfeature/server-sdk
+```
+
+```javascript
+import { OpenFeature } from '@openfeature/server-sdk'
+import tracer from 'dd-trace';
+
+tracer.init({
+  experimental: {
+    flaggingProvider: {
+      enabled: true,
+    }
+  }
+});
+
+OpenFeature.setProvider(tracer.openfeature);
+```
+
+### Accepting default variant until initialization is complete
+
+When you call `setProvider` without waiting, the client returns default values until Remote Configuration loads in the background. This approach keeps your application responsive during startup but may serve defaults for early requests.
+
+```javascript
+OpenFeature.setProvider(tracer.openfeature);
+const client = OpenFeature.getClient();
+
+app.get('/my-endpoint', (req, res) => {
+  const value = client.getBooleanValue('my-flag', false);
+  if (value) {
+    res.send('feature enabled!');
+  } else {
+    res.send('feature disabled!');
+  }
+});
+```
+
+### Waiting for initialization before evaluating a flag
+
+Use `setProviderAndWait` to ensure the provider fully initializes before evaluating flags. This guarantees that flag evaluations use actual configuration data rather than defaults, at the cost of delaying requests during initialization.
+
+```javascript
+const initializationPromise = OpenFeature.setProviderAndWait(tracer.openfeature);
+const client = OpenFeature.getClient();
+
+app.get('/my-endpoint', async (req, res) => {
+  await initializationPromise;
+
+  OpenFeature.setContext({
+    userID: req.session?.userID,
+    companyID: req.session?.companyID
+  });
+
+  const value = client.getBooleanValue('my-flag', false);
+  if (value) {
+    res.send('feature enabled!');
+  } else {
+    res.send('feature disabled!');
+  }
+});
+```
+
+## Set the evaluation context
+
+Define who or what the flag evaluation applies to using an `EvaluationContext`. The evaluation context can include user or session information used to determine which flag variations should be returned. Call `OpenFeature.setContext` method before evaluating flags to ensure proper targeting.
+
+## Evaluating flags
+
+After creating the `OpenFeature` client as described in the previous section, you can start reading flag values throughout your app. Flag evaluation uses locally cached data, so no network requests occur when evaluating flags.
+
+Each flag is identified by a _key_ (a unique string) and can be evaluated with a _typed getter_ that returns a value of the expected type. If the flag doesn't exist or cannot be evaluated, the SDK returns the provided default value.
+
+### Boolean flags
+
+Use `getBooleanValue()` for flags that represent on/off or true/false conditions. Optionally set the context for specific targeting rules.
+
+```javascript
+OpenFeature.setContext({
+  userID: req.session?.userID,
+  companyID: req.session?.companyID
+});
+
+const isNewCheckoutEnabled = client.getBooleanValue(
+    'new-checkout-flow', // flag key
+    false, // default value
+);
+
+if (isNewCheckoutEnabled) {
+    showNewCheckoutFlow();
+} else {
+    showLegacyCheckout();
+}
+```
+
+### String flags
+
+Use `getStringValue()` for flags that select between multiple variants or configuration strings. For example:
+
+```javascript
+OpenFeature.setContext({
+  userID: req.session?.userID,
+  companyID: req.session?.companyID
+});
+
+const theme = client.getStringValue(
+  'ui-theme', // flag key
+  'light', // default value
+);
+
+switch (theme) {
+  case 'light':
+      setLightTheme();
+      break;
+  case 'dark':
+      setDarkTheme();
+      break;
+  case 'blue':
+      setBlueTheme();
+      break;
+  default:
+      setLightTheme();
+}
+```
+
+### Number flags
+
+For number flags, use `getNumberValue()`. This is appropriate when a feature depends on a numeric parameter such as a limit, percentage, or multiplier:
+
+```javascript
+OpenFeature.setContext({
+  userID: req.session?.userID,
+  companyID: req.session?.companyID
+});
+
+const maxItems = client.getNumberValue(
+    'max-cart-items', // flag key
+    20, // default value
+);
+
+const priceMultiplier = client.getNumberValue(
+    'pricing-multiplier', // flag key
+    1.3, // default value
+);
+```
+
+### Object flags
+
+For structured JSON data, use `getObjectValue()`. This method returns an `object`, which can represent primitives, arrays, or dictionaries. Object flags are useful for remote configuration scenarios where multiple properties need to be provided together.
+
+```javascript
+OpenFeature.setContext({
+  userID: req.session?.userID,
+  companyID: req.session?.companyID
+});
+
+const defaultConfig = {
+  color: '#00A3FF',
+  fontSize: 14,
+};
+const config = client.getObjectValue('ui-config', defaultConfig);
+```
+
+### Flag evaluation details
+
+When you need more than just the flag value, use the `get<Type>Details` functions. These methods return both the evaluated value and metadata explaining the evaluation:
+
+* `getBooleanDetails() -> EvaluationDetails<boolean>`
+* `getStringDetails() -> EvaluationDetails<string>`
+* `getNumberDetails() -> EvaluationDetails<number>`
+* `getObjectDetails() -> EvaluationDetails<JsonValue>`
+
+For example:
+
+```javascript
+const details = client.getStringDetails('paywall-layout', 'control');
+
+console.log(details.value);        // Evaluated value (for example: "A", "B", or "control")
+console.log(details.variant);      // Variant name, if applicable
+console.log(details.reason);       // Description of why this value was chosen (for example: "TARGETING_MATCH")
+console.log(details.errorCode);    // The error that occurred during evaluation, if any
+console.log(details.errorMessage); // A more detailed message of the error that occurred, if any
+console.log(details.flagMetadata); // Additional information about the evaluation
+```