🐛 Harden debugger error formatting for hostile values by watson · Pull Request #4740 · DataDog/browser-sdk

watson · 2026-06-07T05:15:49Z

Motivation

Live Debugger runs inside instrumented application control flow, so debugger formatting must tolerate arbitrary JavaScript values without throwing while building snapshots.

JavaScript can throw any value, not just Error instances. Debugger conditions, templates, and captures can also encounter cross-realm Errors, hostile getters, or values that cannot be coerced to strings.

Changes

This PR hardens debugger error formatting and capture behavior:

normalize debugger throwables from unknown values instead of assuming Error
preserve message and parsed stacktrace for Error values, including cross-realm Errors
format non-Error values safely and keep an empty stacktrace for the current Live Debugger UI contract
safely format condition and template evaluation errors from unknown thrown values
inject a scoped $dd_format_error helper into generated template code catch blocks
capture cross-realm Errors and Error properties whose getters throw
add shared browser-core helpers for safe String() and sanitized JSON formatting

Test instructions

Run the focused unit tests:

yarn test:unit --spec packages/browser-debugger/src/domain/api.spec.ts --spec packages/browser-debugger/src/domain/capture.spec.ts --spec packages/browser-debugger/src/domain/condition.spec.ts --spec packages/browser-debugger/src/domain/template.spec.ts --spec packages/browser-debugger/src/domain/expression.spec.ts --spec packages/browser-debugger/src/domain/probes.spec.ts --spec packages/browser-core/src/domain/error/error.spec.ts --spec packages/browser-core/src/tools/serialisation/stringify.spec.ts

Checklist

Tested locally
Tested on staging
Added unit tests for this change.
Added e2e/integration tests for this change.
Updated documentation and/or relevant AGENTS.md file

watson · 2026-06-07T05:16:04Z

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cit-pr-commenter-54b7da · 2026-06-07T05:17:04Z

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	171.88 KiB	171.90 KiB	+20 B	+0.01%	✅
Rum Profiler	7.88 KiB	7.88 KiB	0 B	0.00%	✅
Rum Recorder	21.21 KiB	21.21 KiB	0 B	0.00%	✅
Logs	54.34 KiB	54.36 KiB	+20 B	+0.04%	✅
Rum Slim	129.72 KiB	129.74 KiB	+20 B	+0.02%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅

datadog-prod-us1-5 · 2026-06-07T05:18:39Z

Tests

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 72.22%
• Overall Coverage: 76.77% (-0.04%)

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: f1febfb | Docs | Datadog PR Page | Give us feedback!}

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chatgpt-codex-connector · 2026-06-08T12:50:57Z

      if (!fn) {
        // eslint-disable-next-line no-new-func, @typescript-eslint/no-implied-eval
-        fn = new Function('$dd_inspect', ...contextKeys, fnBodyTemplate) as (...args: any[]) => unknown[]
+        fn = new Function('$dd_inspect', '$dd_format_error', ...contextKeys, fnBodyTemplate) as (


Avoid shadowing the template error helper

When the instrumented function has an argument or local named $dd_format_error and any template segment throws during evaluation, this helper parameter is duplicated by ...contextKeys and the customer value shadows the formatter passed at call time. Because debugger expression identifiers can start with $, the catch generated in compileSegments() then calls the customer value as a function and the whole template falls back to a TypeError instead of reporting the original segment error; use a collision-free binding or filter/rename helper parameters before appending context keys.

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I'll fix this in another PR

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JavaScript allows throwing arbitrary values, but the debugger hook typed thrown values as Error and read message/stack unconditionally. Type thrown values as unknown, safely format non-Error throwables, and keep an empty stacktrace for the current Live Debugger UI contract. Reuse the shared browser-core isError helper for cross-realm Error detection and make it non-throwing for hostile toStringTag getters. Add unit tests covering thrown strings, cross-realm Errors, and values that cannot be coerced to strings.

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chatgpt-codex-connector · 2026-06-08T18:08:18Z

+  if (isError(obj)) {
    return captureError(obj, depth, maxReferenceDepth, maxCollectionSize, maxFieldCount, maxLength, ctx)


Guard Error stack/cause captures

When this branch now accepts Error-like objects via isError() (for example { [Symbol.toStringTag]: 'Error', stack: new Proxy({}, { ownKeys() { throw ... } }) }), captureError() only guards the initial stack/cause getter and then calls captureValue() outside that guard. If the returned stack/cause value throws while being captured, the debugger hook can still throw; before this change the same Error-like object used the generic property path, whose per-property try covered the nested capture as well.

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watson requested review from a team as code owners June 7, 2026 05:15

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Comment thread packages/browser-debugger/src/domain/api.ts Outdated

Comment thread packages/browser-debugger/src/domain/api.ts Outdated

watson changed the base branch from main to graphite-base/4740 June 7, 2026 05:21

watson force-pushed the watson/support-non-error-thrown branch from 03b6d4c to 9c9b8ac Compare June 7, 2026 05:21

watson changed the base branch from graphite-base/4740 to watson/cleanup-tests June 7, 2026 05:21

This was referenced Jun 7, 2026

✅ Simplify debugger probe test fixtures #4736

Merged

✅ Simplify debugger API spec call sites #4741

Draft

watson force-pushed the watson/support-non-error-thrown branch from 9c9b8ac to f32c7a4 Compare June 7, 2026 05:43

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Comment thread packages/browser-debugger/src/domain/api.ts Outdated

Base automatically changed from watson/cleanup-tests to main June 8, 2026 06:03

watson force-pushed the watson/support-non-error-thrown branch 2 times, most recently from ae7762d to 9b05c9e Compare June 8, 2026 12:21

watson changed the title ~~🐛 Handle non-Error debugger throw values~~ 🐛 Harden debugger error formatting for hostile values Jun 8, 2026

watson force-pushed the watson/support-non-error-thrown branch from 9b05c9e to f6260d9 Compare June 8, 2026 12:23

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Comment thread packages/browser-core/src/tools/serialisation/stringify.ts

Comment thread packages/browser-debugger/src/domain/template.ts Outdated

watson force-pushed the watson/support-non-error-thrown branch from f6260d9 to b4c641f Compare June 8, 2026 12:45

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watson force-pushed the watson/support-non-error-thrown branch from b4c641f to eb2aef1 Compare June 8, 2026 13:15

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Comment thread packages/browser-debugger/src/domain/api.ts Outdated

Comment thread packages/browser-debugger/src/domain/condition.ts Outdated

Comment thread packages/browser-debugger/src/domain/template.ts Outdated

watson force-pushed the watson/support-non-error-thrown branch from eb2aef1 to f4a6786 Compare June 8, 2026 17:57

🐛 Harden debugger error formatting for hostile values

753205e

watson force-pushed the watson/support-non-error-thrown branch from f4a6786 to 753205e Compare June 8, 2026 18:00

chatgpt-codex-connector Bot reviewed Jun 8, 2026

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Remove Error.isError

f1febfb

		if (isError(obj)) {
		return captureError(obj, depth, maxReferenceDepth, maxCollectionSize, maxFieldCount, maxLength, ctx)

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