feat: Add support for client-side prerequisite events.

src/__tests__/LDClient-events-test.js

@@ -1,3 +1,4 @@
+// @ts-nocheck
 import * as messages from '../messages';
 
 import { withCloseable, sleepAsync } from 'launchdarkly-js-test-helpers';
@@ -253,6 +254,81 @@ describe('LDClient events', () => {
     });
   });
 
+  it('sends events for prerequisites', async () => {
+    const initData = makeBootstrap({
+      'is-prereq': {
+        value: true,
+        variation: 1,
+        reason: {
+          kind: 'FALLTHROUGH',
+        },
+        version: 1,
+        trackEvents: true,
+        trackReason: true,
+      },
+      'has-prereq-depth-1': {
+        value: true,
+        variation: 0,
+        prerequisites: ['is-prereq'],
+        reason: {
+          kind: 'FALLTHROUGH',
+        },
+        version: 4,
+        trackEvents: true,
+        trackReason: true,
+      },
+      'has-prereq-depth-2': {
+        value: true,
+        variation: 0,
+        prerequisites: ['has-prereq-depth-1'],
+        reason: {
+          kind: 'FALLTHROUGH',
+        },
+        version: 5,
+        trackEvents: true,
+        trackReason: true,
+      },
+    });
+    await withClientAndEventProcessor(user, { bootstrap: initData }, async (client, ep) => {
+      await client.waitForInitialization(5);
+      client.variation('has-prereq-depth-2', false);
+
+      // An identify event and 3 feature events.
+      expect(ep.events.length).toEqual(4);
+      expectIdentifyEvent(ep.events[0], user);
+      expect(ep.events[1]).toMatchObject({
+        kind: 'feature',
+        key: 'is-prereq',
+        variation: 1,
+        value: true,
+        version: 1,
+        reason: {
+          kind: 'FALLTHROUGH',
+        },
+      });
+      expect(ep.events[2]).toMatchObject({
+        kind: 'feature',
+        key: 'has-prereq-depth-1',
+        variation: 0,
+        value: true,
+        version: 4,
+        reason: {
+          kind: 'FALLTHROUGH',
+        },
+      });
+      expect(ep.events[3]).toMatchObject({
+        kind: 'feature',
+        key: 'has-prereq-depth-2',
+        variation: 0,
+        value: true,
+        version: 5,
+        reason: {
+          kind: 'FALLTHROUGH',
+        },
+      });
+    });
+  });
+
   it('sends a feature event on receiving a new flag value', async () => {
     const oldFlags = { foo: { value: 'a', variation: 1, version: 2, flagVersion: 2000 } };
     const newFlags = { foo: { value: 'b', variation: 2, version: 3, flagVersion: 2001 } };

src/index.js

@@ -308,9 +308,10 @@ function initialize(env, context, specifiedOptions, platform, extraOptionDefs) {
 
   function variationDetailInternal(key, defaultValue, sendEvent, includeReasonInEvent, isAllFlags) {
     let detail;
+    let flag;
 
     if (flags && utils.objectHasOwnProperty(flags, key) && flags[key] && !flags[key].deleted) {
-      const flag = flags[key];
+      flag = flags[key];
       detail = getFlagDetail(flag);
       if (flag.value === null || flag.value === undefined) {
         detail.value = defaultValue;
@@ -320,6 +321,12 @@ function initialize(env, context, specifiedOptions, platform, extraOptionDefs) {
     }
 
     if (sendEvent) {
+      // An event will be send for each of these by virtue of sending events for all flags.
+      if (!isAllFlags) {
+        flag?.prerequisites?.forEach(key => {
+          variation(key, undefined);
+        });
+      }
       sendFlagEvent(key, detail, defaultValue, includeReasonInEvent);
     }
 

typings.d.ts

@@ -552,7 +552,7 @@ declare module 'launchdarkly-js-sdk-common' {
 
   /**
    * Describes the reason that a flag evaluation produced a particular value. This is
-   * part of the {@link LDEvaluationDetail} object returned by {@link LDClient.variationDetail]].
+   * part of the {@link LDEvaluationDetail} object returned by {@link LDClient.variationDetail}.
    */
   export interface LDEvaluationReason {
     /**