🐛 add support for arbitrary Error causes (#3860)

✨ accept non-error as error cause

👷 remove comments

🐛 format

🐛 remove strange edge case for safari

🐛 pr-feedback: type undefined for custom errors

🐛 pr-feedback: refactor error base

Co-authored-by: adrian.delarosa <adrian.delarosa@datadoghq.com>

Author: mormubis

packages/core/src/domain/error/error.spec.ts

@@ -234,16 +234,6 @@ describe('flattenErrorCauses', () => {
     expect(errorCauses).toEqual(undefined)
   })
 
-  it('should return undefined if cause is not of type Error', () => {
-    const error = new Error('foo') as ErrorWithCause
-    const nestedError = { biz: 'buz', cause: new Error('boo') } as unknown as Error
-
-    error.cause = nestedError
-
-    const errorCauses = flattenErrorCauses(error, ErrorSource.LOGGER)
-    expect(errorCauses?.length).toEqual(undefined)
-  })
-
   it('should use error to extract stack trace', () => {
     const error = new Error('foo') as ErrorWithCause
 
@@ -259,6 +249,96 @@ describe('flattenErrorCauses', () => {
     const errorCauses = flattenErrorCauses(error, ErrorSource.LOGGER)
     expect(errorCauses?.length).toEqual(10)
   })
+
+  describe('with non-Error values', () => {
+    it('should handle string cause with consistent structure', () => {
+      const error = new Error('main') as ErrorWithCause
+      error.cause = 'string cause'
+
+      const causes = flattenErrorCauses(error, ErrorSource.CUSTOM)
+      expect(causes?.length).toBe(1)
+      expect(causes?.[0]).toEqual({
+        message: '"string cause"', // JSON stringified
+        source: ErrorSource.CUSTOM,
+        type: undefined,
+        stack: undefined,
+      })
+    })
+
+    it('should handle object cause with consistent structure', () => {
+      const error = new Error('main') as ErrorWithCause
+      error.cause = { code: 'ERR_001', details: 'Invalid input' }
+
+      const causes = flattenErrorCauses(error, ErrorSource.CUSTOM)
+      expect(causes?.length).toBe(1)
+      expect(causes?.[0]).toEqual({
+        message: '{"code":"ERR_001","details":"Invalid input"}',
+        source: ErrorSource.CUSTOM,
+        type: undefined,
+        stack: undefined,
+      })
+    })
+
+    it('should handle number cause with consistent structure', () => {
+      const error = new Error('main') as ErrorWithCause
+      error.cause = 42
+
+      const causes = flattenErrorCauses(error, ErrorSource.CUSTOM)
+      expect(causes?.length).toBe(1)
+      expect(causes?.[0]).toEqual({
+        message: '42',
+        source: ErrorSource.CUSTOM,
+        type: undefined,
+        stack: undefined,
+      })
+    })
+
+    it('should handle mixed Error and non-Error chain', () => {
+      const error1 = new Error('first') as ErrorWithCause
+      const error2 = new Error('second') as ErrorWithCause
+      error1.cause = error2
+      error2.cause = { code: 'ERR_ROOT' }
+
+      const causes = flattenErrorCauses(error1, ErrorSource.CUSTOM)
+      expect(causes?.length).toBe(2)
+
+      // First cause: Error with full structure
+      expect(causes?.[0].message).toBe('second')
+      expect(causes?.[0].type).toBe('Error')
+      expect(causes?.[0].stack).toContain('Error')
+
+      // Second cause: Object with normalized structure
+      expect(causes?.[1]).toEqual({
+        message: '{"code":"ERR_ROOT"}',
+        source: ErrorSource.CUSTOM,
+        type: undefined,
+        stack: undefined,
+      })
+    })
+
+    it('should stop chain after non-Error cause', () => {
+      const error = new Error('main') as ErrorWithCause
+      error.cause = { value: 'data', cause: new Error('ignored') }
+
+      const causes = flattenErrorCauses(error, ErrorSource.CUSTOM)
+      expect(causes?.length).toBe(1)
+      // The entire object is captured, nested cause is sanitized
+      expect(causes?.[0].message).toContain('"value":"data"')
+      expect(causes?.[0].type).toBeUndefined()
+    })
+
+    it('should handle null cause', () => {
+      const error = new Error('main') as ErrorWithCause
+      error.cause = null
+      expect(flattenErrorCauses(error, ErrorSource.CUSTOM)).toBeUndefined()
+    })
+
+    it('should handle undefined cause', () => {
+      const error = new Error('main') as ErrorWithCause
+      error.cause = undefined
+      expect(flattenErrorCauses(error, ErrorSource.CUSTOM)).toBeUndefined()
+    })
+  })
 })
 
 describe('isError', () => {

packages/core/src/domain/error/error.ts

@@ -21,6 +21,32 @@ interface RawErrorParams {
   handling: ErrorHandling
 }
 
+function computeErrorBase({
+  originalError,
+  stackTrace,
+  source,
+  useFallbackStack = true,
+  nonErrorPrefix,
+}: {
+  originalError: unknown
+  stackTrace?: StackTrace
+  source: ErrorSource
+  useFallbackStack?: boolean
+  nonErrorPrefix?: NonErrorPrefix
+}) {
+  const isErrorInstance = isError(originalError)
+  if (!stackTrace && isErrorInstance) {
+    stackTrace = computeStackTrace(originalError)
+  }
+
+  return {
+    source,
+    type: stackTrace ? stackTrace.name : undefined,
+    message: computeMessage(stackTrace, isErrorInstance, nonErrorPrefix, originalError),
+    stack: stackTrace ? toStackTraceString(stackTrace) : useFallbackStack ? NO_ERROR_STACK_PRESENT_MESSAGE : undefined,
+  }
+}
+
 export function computeRawError({
   stackTrace,
   originalError,
@@ -32,22 +58,16 @@ export function computeRawError({
   source,
   handling,
 }: RawErrorParams): RawError {
-  const isErrorInstance = isError(originalError)
-  if (!stackTrace && isErrorInstance) {
-    stackTrace = computeStackTrace(originalError)
-  }
+  const errorBase = computeErrorBase({ originalError, stackTrace, source, useFallbackStack, nonErrorPrefix })
 
   return {
     startClocks,
-    source,
     handling,
     handlingStack,
     componentStack,
     originalError,
-    type: stackTrace ? stackTrace.name : undefined,
-    message: computeMessage(stackTrace, isErrorInstance, nonErrorPrefix, originalError),
-    stack: stackTrace ? toStackTraceString(stackTrace) : useFallbackStack ? NO_ERROR_STACK_PRESENT_MESSAGE : undefined,
-    causes: isErrorInstance ? flattenErrorCauses(originalError as ErrorWithCause, source) : undefined,
+    ...errorBase,
+    causes: isError(originalError) ? flattenErrorCauses(originalError as ErrorWithCause, source) : undefined,
     fingerprint: tryToGetFingerprint(originalError),
     context: tryToGetErrorContext(originalError),
   }
@@ -56,15 +76,17 @@ export function computeRawError({
 function computeMessage(
   stackTrace: StackTrace | undefined,
   isErrorInstance: boolean,
-  nonErrorPrefix: NonErrorPrefix,
+  nonErrorPrefix: NonErrorPrefix | undefined,
   originalError: unknown
 ) {
   // Favor stackTrace message only if tracekit has really been able to extract something meaningful (message + name)
   // TODO rework tracekit integration to avoid scattering error building logic
   return stackTrace?.message && stackTrace?.name
     ? stackTrace.message
     : !isErrorInstance
-      ? `${nonErrorPrefix} ${jsonStringify(sanitize(originalError))!}`
+      ? nonErrorPrefix
+        ? `${nonErrorPrefix} ${jsonStringify(sanitize(originalError))!}`
+        : jsonStringify(sanitize(originalError))!
       : 'Empty message'
 }
 
@@ -87,17 +109,20 @@ export function isError(error: unknown): error is Error {
 }
 
 export function flattenErrorCauses(error: ErrorWithCause, parentSource: ErrorSource): RawErrorCause[] | undefined {
-  let currentError = error
   const causes: RawErrorCause[] = []
-  while (isError(currentError?.cause) && causes.length < 10) {
-    const stackTrace = computeStackTrace(currentError.cause)
-    causes.push({
-      message: currentError.cause.message,
+  let currentCause = error.cause
+
+  while (currentCause !== undefined && currentCause !== null && causes.length < 10) {
+    const causeBase = computeErrorBase({
+      originalError: currentCause,
       source: parentSource,
-      type: stackTrace?.name,
-      stack: stackTrace && toStackTraceString(stackTrace),
+      useFallbackStack: false,
     })
-    currentError = currentError.cause
+
+    causes.push(causeBase)
+
+    currentCause = isError(currentCause) ? (currentCause as ErrorWithCause).cause : undefined
   }
+
   return causes.length ? causes : undefined
 }

packages/rum-core/src/domain/error/errorCollection.spec.ts

@@ -126,6 +126,27 @@ describe('error collection', () => {
       expect(error?.causes?.[1].source).toEqual(ErrorSource.CUSTOM)
     })
 
+    it('should extract non-Error causes with consistent structure', () => {
+      setupErrorCollection()
+      const error = new Error('RSA key generation failed') as ErrorWithCause
+      error.cause = { code: 'NonInteger', values: [3.14, 2.71] }
+
+      addError({
+        error,
+        handlingStack: 'Error: handling',
+        startClocks: { relative: 1234 as RelativeTime, timeStamp: 123456789 as TimeStamp },
+      })
+
+      const { error: rumError } = rawRumEvents[0].rawRumEvent as RawRumErrorEvent
+      expect(rumError.causes?.length).toBe(1)
+      expect(rumError.causes?.[0]).toEqual({
+        message: '{"code":"NonInteger","values":[3.14,2.71]}',
+        source: ErrorSource.CUSTOM,
+        type: undefined,
+        stack: undefined,
+      })
+    })
+
     it('should extract fingerprint from error', () => {
       setupErrorCollection()