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feat: send $device_id on feature flag requests (#2708)
* send device_id on feature flag requests * add changeset * include in mock * use isNull and isUndefined instead of direct checks * update functional tests
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‎.changeset/dirty-news-drive.md‎

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@@ -0,0 +1,5 @@
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---
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'posthog-js': patch
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---
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Include $device_id when fetching feature flags

‎packages/browser/functional_tests/feature-flags.test.ts‎

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@@ -27,6 +27,7 @@ describe('FunctionalTests / Feature Flags', () => {
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expect(getRequests(token)['/flags/']).toEqual([
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// This is the initial call to the flags endpoint on PostHog init.
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{
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$device_id: anonymousId,
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distinct_id: anonymousId,
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person_properties: {},
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groups: {},
@@ -53,6 +54,7 @@ describe('FunctionalTests / Feature Flags', () => {
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// `identify()`.
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{
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$anon_distinct_id: anonymousId,
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$device_id: anonymousId,
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distinct_id: 'test-id',
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person_properties: {
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$initial__kx: null,
@@ -102,6 +104,7 @@ describe('FunctionalTests / Feature Flags', () => {
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expect(getRequests(token)['/flags/']).toEqual([
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// This is the initial call to the flags endpoint on PostHog init.
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{
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$device_id: anonymousId,
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distinct_id: anonymousId,
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person_properties: {},
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groups: {},
@@ -125,6 +128,7 @@ describe('FunctionalTests / Feature Flags', () => {
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// `identify()`.
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{
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$anon_distinct_id: anonymousId,
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$device_id: anonymousId,
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distinct_id: 'test-id',
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groups: {},
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person_properties: {
@@ -172,6 +176,7 @@ describe('FunctionalTests / Feature Flags', () => {
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await waitFor(() => {
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expect(getRequests(token)['/flags/']).toEqual([
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{
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$device_id: anonymousId,
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distinct_id: 'test-id',
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groups: {},
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person_properties: {
@@ -221,6 +226,7 @@ describe('FunctionalTests / Feature Flags', () => {
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expect(getRequests(token)['/flags/']).toEqual([
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// This is the initial call to the flags endpoint on PostHog init.
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{
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$device_id: anonymousId,
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distinct_id: anonymousId,
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person_properties: {},
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groups: {},
@@ -248,6 +254,7 @@ describe('FunctionalTests / Feature Flags', () => {
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expect(getRequests(token)['/flags/']).toEqual([
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{
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$anon_distinct_id: anonymousId,
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$device_id: anonymousId,
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distinct_id: 'test-id',
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groups: {},
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person_properties: {
@@ -304,6 +311,7 @@ describe('FunctionalTests / Feature Flags', () => {
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// This is the initial call to the flags endpoint on PostHog init, with all info added from `loaded`.
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{
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$anon_distinct_id: 'anon-id',
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$device_id: 'anon-id',
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distinct_id: 'test-id',
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groups: { playlist: 'id:77' },
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person_properties: {

‎packages/browser/playwright/mocked/flags.spec.ts‎

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@@ -68,6 +68,7 @@ test.describe('flags', () => {
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expect(flagsPayload).toEqual({
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token: 'test token',
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distinct_id: 'new-id',
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$device_id: flagsPayload.$device_id,
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person_properties: {
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$initial__kx: null,
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$initial_current_url: 'http://localhost:2345/playground/cypress/index.html',

‎packages/browser/src/__tests__/featureflags.test.ts‎

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@@ -1169,6 +1169,136 @@ describe('featureflags', () => {
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})
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})
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describe('device_id in flags requests', () => {
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beforeEach(() => {
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// Clear persistence before each test in this suite
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instance.persistence.unregister('$device_id')
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instance.persistence.unregister('$stored_person_properties')
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instance.persistence.unregister('$stored_group_properties')
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instance._send_request = jest.fn().mockImplementation(({ callback }) =>
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callback({
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statusCode: 200,
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json: {
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featureFlags: {
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first: 'variant-1',
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second: true,
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},
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},
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})
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)
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})
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afterEach(() => {
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// Clean up after each test
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instance.persistence.unregister('$device_id')
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instance.persistence.unregister('$stored_person_properties')
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instance.persistence.unregister('$stored_group_properties')
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})
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it('should include device_id in flags request when available', () => {
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instance.persistence.register({
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$device_id: 'test-device-uuid-123',
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})
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featureFlags.reloadFeatureFlags()
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jest.runAllTimers()
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expect(instance._send_request).toHaveBeenCalledTimes(1)
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expect(instance._send_request.mock.calls[0][0].data).toEqual({
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token: 'random fake token',
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distinct_id: 'blah id',
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$device_id: 'test-device-uuid-123',
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person_properties: {},
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})
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})
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it('should omit device_id when it is null (cookieless mode)', () => {
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instance.persistence.register({
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$device_id: null,
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})
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featureFlags.reloadFeatureFlags()
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jest.runAllTimers()
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expect(instance._send_request).toHaveBeenCalledTimes(1)
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expect(instance._send_request.mock.calls[0][0].data).toEqual({
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token: 'random fake token',
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distinct_id: 'blah id',
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person_properties: {},
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})
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expect(instance._send_request.mock.calls[0][0].data).not.toHaveProperty('$device_id')
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})
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it('should omit device_id when it is undefined', () => {
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// Don't register device_id at all
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featureFlags.reloadFeatureFlags()
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jest.runAllTimers()
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expect(instance._send_request).toHaveBeenCalledTimes(1)
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expect(instance._send_request.mock.calls[0][0].data).toEqual({
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token: 'random fake token',
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distinct_id: 'blah id',
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person_properties: {},
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})
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expect(instance._send_request.mock.calls[0][0].data).not.toHaveProperty('$device_id')
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})
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it('should include device_id along with $anon_distinct_id on identify', () => {
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instance.persistence.register({
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$device_id: 'device-uuid-456',
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})
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featureFlags.setAnonymousDistinctId('anon_id_789')
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featureFlags.reloadFeatureFlags()
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jest.runAllTimers()
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expect(instance._send_request).toHaveBeenCalledTimes(1)
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expect(instance._send_request.mock.calls[0][0].data).toEqual({
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token: 'random fake token',
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distinct_id: 'blah id',
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$device_id: 'device-uuid-456',
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$anon_distinct_id: 'anon_id_789',
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person_properties: {},
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})
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})
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it('should include device_id with person_properties', () => {
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instance.persistence.register({
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$device_id: 'device-uuid-999',
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})
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featureFlags.setPersonPropertiesForFlags({ plan: 'pro', beta_tester: true })
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jest.runAllTimers()
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expect(instance._send_request).toHaveBeenCalledTimes(1)
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expect(instance._send_request.mock.calls[0][0].data).toEqual({
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token: 'random fake token',
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distinct_id: 'blah id',
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$device_id: 'device-uuid-999',
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person_properties: { plan: 'pro', beta_tester: true },
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})
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})
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it('should include device_id with group_properties', () => {
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instance.persistence.register({
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$device_id: 'device-uuid-888',
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})
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featureFlags.setGroupPropertiesForFlags({ company: { name: 'Acme', seats: 50 } })
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jest.runAllTimers()
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expect(instance._send_request).toHaveBeenCalledTimes(1)
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expect(instance._send_request.mock.calls[0][0].data).toEqual({
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token: 'random fake token',
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distinct_id: 'blah id',
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$device_id: 'device-uuid-888',
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person_properties: {},
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group_properties: { company: { name: 'Acme', seats: 50 } },
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})
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})
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})
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describe('reloadFeatureFlags', () => {
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beforeEach(() => {
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instance._send_request = jest.fn().mockImplementation(({ callback }) =>

‎packages/browser/src/posthog-featureflags.ts‎

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@@ -24,7 +24,7 @@ import {
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FLAG_CALL_REPORTED,
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} from './constants'
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import { isUndefined, isArray } from '@posthog/core'
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import { isUndefined, isArray, isNull } from '@posthog/core'
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import { createLogger } from './utils/logger'
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import { getTimezone } from './utils/event-utils'
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@@ -398,6 +398,8 @@ export class PostHogFeatureFlags {
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return
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}
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const token = this._instance.config.token
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const deviceId = this._instance.get_property('$device_id')
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const data: Record<string, any> = {
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token: token,
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distinct_id: this._instance.get_distinct_id(),
@@ -410,6 +412,11 @@ export class PostHogFeatureFlags {
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group_properties: this._instance.get_property(STORED_GROUP_PROPERTIES_KEY),
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}
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// Add device_id if available (handle cookieless mode where it's null)
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if (!isNull(deviceId) && !isUndefined(deviceId)) {
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data.$device_id = deviceId
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}
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if (options?.disableFlags || this._instance.config.advanced_disable_feature_flags) {
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data.disable_flags = true
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}

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