Node.js Feature Flags

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Overview

This page describes how to instrument your Node.js application with the Datadog Feature Flags SDK. The Node.js SDK integrates with OpenFeature, an open standard for feature flag management, and uses the Datadog tracer’s Remote Configuration to receive flag updates in real time.

Prerequisites

Before setting up the Node.js Feature Flags SDK, ensure you have:

Installing and initializing

Feature Flagging is provided by Application Performance Monitoring (APM). To integrate APM into your application with feature flagging support, install dd-trace and enable the feature flagging provider. See Tracing Node.js Applications for detailed APM installation instructions.

npm install dd-trace @openfeature/server-sdk

import { OpenFeature } from '@openfeature/server-sdk'
import tracer from 'dd-trace';

tracer.init({
  experimental: {
    flaggingProvider: {
      enabled: true,
    }
  }
});

OpenFeature.setProvider(tracer.openfeature);

Accepting default values before initialization

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.

OpenFeature.setProvider(tracer.openfeature);
const client = OpenFeature.getClient();

app.get('/my-endpoint', async (req, res) => {

  const flagKey = 'my-flag';
  const defaultValue = false;
  const evaluationContext = {
    targetingKey: req.session?.userID ?? 'unknown', // targetingKey is required context
    companyID: req.session?.companyID
  };

  // Note: evaluations are synchronous, but return a Promise type
  //       to follow the OpenFeature provider specifications
  const value = await client.getBooleanValue(flagKey, defaultValue, evaluationContext);

  if (value) {
    res.send('feature enabled!');
  } else {
    res.send('feature disabled!');
  }

});

Waiting for provider initialization

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.

const initializationPromise = OpenFeature.setProviderAndWait(tracer.openfeature);
const client = OpenFeature.getClient();

app.get('/my-endpoint', async (req, res) => {

  const flagKey = 'my-flag';
  const defaultValue = false;
  const evaluationContext = {
    targetingKey: req.session?.userID ?? 'unknown', // targetingKey is required context
    companyID: req.session?.companyID
  };

  // Wait for initialization if necessary
  await initializationPromise;

  // Note: evaluations are synchronous, but return a Promise type
  //       to follow the OpenFeature provider specifications
  const value = await client.getBooleanValue(flagKey, defaultValue, evaluationContext);

  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 the OpenFeature.setContext method before evaluating flags to ensure proper targeting.

Evaluate flags

After creating the OpenFeature client as described in the Installing and initializing 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.

const evaluationContext = {
  targetingKey: req.session?.userID ?? 'unknown',
  companyID: req.session?.companyID
};

const isNewCheckoutEnabled = await client.getBooleanValue(
    'new-checkout-flow', // flag key
    false, // default value
    evaluationContext, // context
);

if (isNewCheckoutEnabled) {
    showNewCheckoutFlow();
} else {
    showLegacyCheckout();
}

String flags

Use getStringValue() for flags that select between multiple variants or configuration strings. For example:

const evaluationContext = {
  targetingKey: req.session?.userID ?? 'unknown',
  companyID: req.session?.companyID
};

const searchAlgorithm = await client.getStringValue(
  'search-algorithm', // flag key
  'basic', // default value
  evaluationContext,
);

switch (searchAlgorithm) {
  case 'basic':
      return basicSearch(query);
  case 'fuzzy':
      return fuzzySearch(query);
  case 'semantic':
      return semanticSearch(query);
  default:
      return basicSearch(query);
}

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:

const evaluationContext = {
  targetingKey: req.session?.userID ?? 'unknown',
  companyID: req.session?.companyID,
};

const maxItems = await client.getNumberValue(
    'max-cart-items', // flag key
    20, // default value
    evaluationContext,
);

const priceMultiplier = await client.getNumberValue(
    'pricing-multiplier', // flag key
    1.3, // default value
    evaluationContext,
);

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.

const defaultConfig = {
  color: '#00A3FF',
  fontSize: 14,
};

const evaluationContext = {
  targetingKey: req.session?.userID ?? 'unknown',
  companyID: req.session?.companyID,
};

const config = await client.getObjectValue('ui-config', defaultConfig, evaluationContext);

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() -> Promise<EvaluationDetails<boolean>>
  • getStringDetails() -> Promise<EvaluationDetails<string>>
  • getNumberDetails() -> Promise<EvaluationDetails<number>>
  • getObjectDetails() -> Promise<EvaluationDetails<JsonValue>>

For example:

const details = await client.getStringDetails('paywall-layout', 'control', evaluationContext);

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

Further reading

Additional helpful documentation, links, and articles:

Server-Side Feature FlagsDOCUMENTATION more more Node.js TracingDOCUMENTATION more more Learn about Application Performance Monitoring (APM)DOCUMENTATION more more