DataDog · mormubis · Mar 18, 2026 · Mar 18, 2026 · Apr 6, 2026 · Apr 6, 2026
@@ -25,6 +25,8 @@ export enum ExperimentalFeature {
   TOO_MANY_REQUESTS_INVESTIGATION = 'too_many_requests_investigation',
   COMPOSED_PATH_SELECTOR = 'composed_path_selector',
   TRACK_RESOURCE_HEADERS = 'track_resource_headers',
+  PARTIAL_VIEW_UPDATES = 'partial_view_updates',
+  PARTIAL_VIEW_UPDATES_NO_CHECKPOINT = 'partial_view_updates_no_checkpoint',
 }
 
 const enabledExperimentalFeatures: Set<ExperimentalFeature> = new Set()

@@ -388,27 +388,31 @@ describe('rum assembly', () => {
   describe('service and version', () => {
     const extraConfigurationOptions = { service: 'default-service', version: 'default-version' }
 
-    Object.values(RumEventType).forEach((eventType) => {
-      it(`should be modifiable for ${eventType}`, () => {
-        const { lifeCycle, serverRumEvents } = setupAssemblyTestWithDefaults({
-          partialConfiguration: {
-            ...extraConfigurationOptions,
-            beforeSend: (event) => {
-              event.service = 'bar'
-              event.version = '0.2.0'
+    // view_update events bypass the assembly pipeline (created post-assembly in startRumBatch)
+    // and are intentionally not modifiable via beforeSend.
+    Object.values(RumEventType)
+      .filter((eventType) => eventType !== RumEventType.VIEW_UPDATE)
+      .forEach((eventType) => {
+        it(`should be modifiable for ${eventType}`, () => {
+          const { lifeCycle, serverRumEvents } = setupAssemblyTestWithDefaults({
+            partialConfiguration: {
+              ...extraConfigurationOptions,
+              beforeSend: (event) => {
+                event.service = 'bar'
+                event.version = '0.2.0'
 
-              return true
+                return true
+              },
             },
-          },
-        })
+          })
 
-        notifyRawRumEvent(lifeCycle, {
-          rawRumEvent: createRawRumEvent(eventType),
+          notifyRawRumEvent(lifeCycle, {
+            rawRumEvent: createRawRumEvent(eventType),
+          })
+          expect((serverRumEvents[0] as RumResourceEvent).service).toBe('bar')
+          expect((serverRumEvents[0] as RumResourceEvent).version).toBe('0.2.0')
         })
-        expect((serverRumEvents[0] as RumResourceEvent).service).toBe('bar')
-        expect((serverRumEvents[0] as RumResourceEvent).version).toBe('0.2.0')
       })
-    })
 
     it('should be added to the event as ddtags', () => {
       const { lifeCycle, serverRumEvents } = setupAssemblyTestWithDefaults({

@@ -49,6 +49,10 @@ export function startRumAssembly(
       ...VIEW_MODIFIABLE_FIELD_PATHS,
       ...ROOT_MODIFIABLE_FIELD_PATHS,
     },
+    // view_update events are created post-assembly in startRumBatch.ts and never go through
+    // this pipeline — they intentionally bypass beforeSend. This entry is required by the
+    // exhaustive type but is never reached in practice.
+    [RumEventType.VIEW_UPDATE]: {},
     [RumEventType.ERROR]: {
       'error.message': 'string',
       'error.stack': 'string',

@@ -30,7 +30,7 @@ export function trackEventCounts({
   }
 
   const subscription = lifeCycle.subscribe(LifeCycleEventType.RUM_EVENT_COLLECTED, (event): void => {
-    if (event.type === 'view' || event.type === 'vital' || !isChildEvent(event)) {
+    if (event.type === 'view' || event.type === 'view_update' || event.type === 'vital' || !isChildEvent(event)) {
       return
     }
     switch (event.type) {

@@ -0,0 +1,116 @@
+import { isEqual, diffMerge } from './viewDiff'
+
+describe('isEqual', () => {
+  it('should return true for identical primitives', () => {
+    expect(isEqual(1, 1)).toBe(true)
+    expect(isEqual('a', 'a')).toBe(true)
+    expect(isEqual(true, true)).toBe(true)
+    expect(isEqual(null, null)).toBe(true)
+    expect(isEqual(undefined, undefined)).toBe(true)
+  })
+
+  it('should return false for different primitives', () => {
+    expect(isEqual(1, 2)).toBe(false)
+    expect(isEqual('a', 'b')).toBe(false)
+    expect(isEqual(true, false)).toBe(false)
+    expect(isEqual(null, undefined)).toBe(false)
+  })
+
+  it('should return true for deeply equal objects', () => {
+    expect(isEqual({ a: 1, b: { c: 2 } }, { a: 1, b: { c: 2 } })).toBe(true)
+  })
+
+  it('should return false for objects with different values', () => {
+    expect(isEqual({ a: 1 }, { a: 2 })).toBe(false)
+  })
+
+  it('should return false for objects with different keys', () => {
+    expect(isEqual({ a: 1 }, { b: 1 })).toBe(false)
+  })
+
+  it('should return true for equal arrays', () => {
+    expect(isEqual([1, 2, 3], [1, 2, 3])).toBe(true)
+  })
+
+  it('should return false for arrays with different lengths', () => {
+    expect(isEqual([1, 2], [1, 2, 3])).toBe(false)
+  })
+
+  it('should return false for arrays with different values', () => {
+    expect(isEqual([1, 2, 3], [1, 2, 4])).toBe(false)
+  })
+
+  it('should return false when comparing array to non-array', () => {
+    expect(isEqual([1], { 0: 1 })).toBe(false)
+  })
+
+  it('should return false for type mismatch', () => {
+    expect(isEqual(1, '1')).toBe(false)
+  })
+})
+
+describe('diffMerge', () => {
+  it('should return undefined when there are no changes', () => {
+    const result = diffMerge({ a: 1, b: 'x' }, { a: 1, b: 'x' })
+    expect(result).toBeUndefined()
+  })
+
+  it('should return changed primitive fields', () => {
+    const result = diffMerge({ a: 1, b: 2 }, { a: 1, b: 1 })
+    expect(result).toEqual({ b: 2 })
+  })
+
+  it('should include new fields not present in lastSent', () => {
+    const result = diffMerge({ a: 1, b: 2 }, { a: 1 })
+    expect(result).toEqual({ b: 2 })
+  })
+
+  it('should set null for deleted keys', () => {
+    const result = diffMerge({ a: 1 }, { a: 1, b: 2 })
+    expect(result).toEqual({ b: null })
+  })
+
+  it('should recursively diff nested objects', () => {
+    const result = diffMerge({ nested: { x: 1, y: 2 } }, { nested: { x: 1, y: 1 } })
+    expect(result).toEqual({ nested: { y: 2 } })
+  })
+
+  it('should return undefined for unchanged nested objects', () => {
+    const result = diffMerge({ nested: { x: 1 } }, { nested: { x: 1 } })
+    expect(result).toBeUndefined()
+  })
+
+  it('should include new nested objects', () => {
+    const result = diffMerge({ nested: { x: 1 } }, {})
+    expect(result).toEqual({ nested: { x: 1 } })
+  })
+
+  describe('replaceKeys option', () => {
+    it('should use full replace strategy for specified keys', () => {
+      const result = diffMerge({ arr: [1, 2, 3] }, { arr: [1, 2] }, { replaceKeys: new Set(['arr']) })
+      expect(result).toEqual({ arr: [1, 2, 3] })
+    })
+
+    it('should not include replace key if unchanged', () => {
+      const result = diffMerge({ arr: [1, 2] }, { arr: [1, 2] }, { replaceKeys: new Set(['arr']) })
+      expect(result).toBeUndefined()
+    })
+  })
+
+  describe('appendKeys option', () => {
+    it('should append only new trailing elements for array keys', () => {
+      const result = diffMerge({ items: [1, 2, 3] }, { items: [1, 2] }, { appendKeys: new Set(['items']) })
+      expect(result).toEqual({ items: [3] })
+    })
+
+    it('should include full array when it first appears', () => {
+      const result = diffMerge({ items: [1, 2] }, {}, { appendKeys: new Set(['items']) })
+      expect(result).toEqual({ items: [1, 2] })
+    })
+
+    it('should not include append key if array has not grown', () => {
+      const result = diffMerge({ items: [1, 2] }, { items: [1, 2] }, { appendKeys: new Set(['items']) })
+      expect(result).toBeUndefined()
+    })
+  })
+})
@@ -0,0 +1,115 @@
+import { isIndexableObject } from '@datadog/browser-core'
+
+/**
+ * Compare two values for deep equality
+ */
+export function isEqual(a: unknown, b: unknown): boolean {
+  if (a === b) {
+    return true
+  }
+
+  if (a === null || typeof a !== 'object' || b === null || typeof b !== 'object') {
+    return a === b
+  }
+
+  // Arrays
+  if (Array.isArray(a) && Array.isArray(b)) {
+    if (a.length !== b.length) {
+      return false
+    }
+    return a.every((val, idx) => isEqual(val, b[idx]))
+  }
+
+  if (Array.isArray(a) || Array.isArray(b)) {
+    return false
+  }
+
+  // Plain objects
+  const aObj = a as Record<string, unknown>
+  const bObj = b as Record<string, unknown>
+  const aKeys = Object.keys(aObj)
+  const bKeys = Object.keys(bObj)
+
+  if (aKeys.length !== bKeys.length) {
+    return false
+  }
+
+  return aKeys.every((key) => bKeys.includes(key) && isEqual(aObj[key], bObj[key]))
+}
+
+/**
+ * Options for controlling diff merge behavior
+ */
+export interface DiffMergeOptions {
+  replaceKeys?: Set<string>
+  appendKeys?: Set<string>
+  ignoreKeys?: Set<string>
+}
+
+/**
+ * MERGE strategy: compare two objects and return an object with only changed fields.
+ * Returns undefined if no changes.
+ *
+ * Default strategy is REPLACE (isEqual check). Exceptions:
+ * - Both values are plain objects and key is not in replaceKeys: recurse (MERGE), sub-paths
+ * (e.g. 'view.custom_timings') are propagated to the recursive call.
+ * - Both values are arrays and key is in appendKeys: include only new trailing elements (APPEND)
+ */
+export function diffMerge(
+  current: Record<string, unknown>,
+  lastSent: Record<string, unknown>,
+  options?: DiffMergeOptions
+): Record<string, unknown> | undefined {
+  const result: Record<string, unknown> = {}
+  const replaceKeys = options?.replaceKeys ?? new Set<string>()
+  const appendKeys = options?.appendKeys ?? new Set<string>()
+  const ignoreKeys = options?.ignoreKeys ?? new Set<string>()
+
+  for (const key of Object.keys(current)) {
+    if (ignoreKeys.has(key)) {
+      continue
+    }
+
+    const currentVal = current[key]
+    const lastSentVal = lastSent[key]
+
+    if (!replaceKeys.has(key) && isIndexableObject(currentVal) && isIndexableObject(lastSentVal)) {
+      // Both are plain objects and not marked for replace: recurse (MERGE)
+      const nestedDiff = diffMerge(currentVal, lastSentVal, {
+        replaceKeys: extractSubPaths(replaceKeys, key),
+        appendKeys: extractSubPaths(appendKeys, key),
+        ignoreKeys: extractSubPaths(ignoreKeys, key),
+      })
+      if (nestedDiff) {
+        result[key] = nestedDiff
+      }
+    } else if (appendKeys.has(key) && Array.isArray(currentVal) && Array.isArray(lastSentVal)) {
+      // Array in appendKeys: include only new trailing elements (APPEND)
+      if (currentVal.length > lastSentVal.length) {
+        result[key] = currentVal.slice(lastSentVal.length)
+      }
+    } else if (!isEqual(currentVal, lastSentVal)) {
+      // Default: replace the whole value (REPLACE)
+      result[key] = currentVal
+    }
+  }
+
+  // Deleted keys: present in lastSent but not in current
+  for (const key of Object.keys(lastSent)) {
+    if (!(key in current)) {
+      result[key] = null
+    }
+  }
+
+  return Object.keys(result).length > 0 ? result : undefined
+}
+
+function extractSubPaths(keys: Set<string>, prefix: string): Set<string> {
+  const result = new Set<string>()
+  for (const key of keys) {
+    if (key.startsWith(`${prefix}.`)) {
+      result.add(key.slice(prefix.length + 1))
+    }
+  }
+  return result
+}
@@ -18,6 +18,7 @@ import type {
   RumLongTaskEvent,
   RumResourceEvent,
   RumViewEvent,
+  RumViewUpdateEvent,
   RumVitalEvent,
 } from './rumEvent.types'
 
@@ -26,6 +27,7 @@ export const RumEventType = {
   ERROR: 'error',
   LONG_TASK: 'long_task',
   VIEW: 'view',
+  VIEW_UPDATE: 'view_update',
   RESOURCE: 'resource',
   VITAL: 'vital',
 } as const
@@ -34,6 +36,7 @@ export type RumEventType = (typeof RumEventType)[keyof typeof RumEventType]
 
 export type AssembledRumEvent = (
   | RumViewEvent
+  | RumViewUpdateEvent
   | RumActionEvent
   | RumResourceEvent
   | RumErrorEvent
@@ -183,6 +186,19 @@ export interface RawRumViewEvent {
   }
 }
 
+export interface RawRumViewUpdateEvent {
+  date: TimeStamp
+  type: typeof RumEventType.VIEW_UPDATE
+  view: Partial<RawRumViewEvent['view']>
+  _dd: Partial<RawRumViewEvent['_dd']> & {
+    document_version: number
+  }
+  display?: Partial<ViewDisplay>
+  privacy?: RawRumViewEvent['privacy']
+  device?: RawRumViewEvent['device']
+  feature_flags?: Context
+}
+
 interface ViewDisplay {
   scroll: {
     max_depth?: number
@@ -410,6 +426,7 @@ export type RawRumEvent =
   | RawRumErrorEvent
   | RawRumResourceEvent
   | RawRumViewEvent
+  | RawRumViewUpdateEvent
   | RawRumLongTaskEvent
   | RawRumLongAnimationFrameEvent
   | RawRumActionEvent