✅ Introduce a new way to mock values in unit tests by BenoitZugmeyer · Pull Request #4129 · DataDog/browser-sdk

BenoitZugmeyer · 2026-01-28T13:46:00Z

Motivation

This is a proposal to allow mocking values (mostly functions) within tests.

The current way to mock values used within a function is to pass them as argument of that function. For example:

// foo.ts
import { startBar } from 'bar'

function createFoo(startBarImpl = startBar) {
  const bar = startBarImpl()
}


// foo.spec.ts
it('create foo', () => {
  createFoo(mockStartBar)
})

However, forces to have unecessary extra arguments in functions. Impls also need to be propagated within subfunctions:

// foo.ts
function createFoo(startBarImpl = startBar) {
  const baz = trackBaz(startBarImpl)
}

// baz.ts
function trackBaz(startBarImpl = startBar) {
}

This PR introduce a new way to mock values that is similar to a lightweight dependency injection system. Values that can be mocked should be wrapped in a mocked() call when used, and tests can use mockValue() to replace the value or mockWithSpy() to replace it with a jasmine spy. The example above would be implemented as:

// foo.ts
import { startBar } from 'bar'

function createFoo() {
  const bar = mockable(startBar)()
}

// foo.spec.ts
it('create foo', () => {
  replaceMockable(startBar, mockStartBar)
  // or
  replaceMockableWithSpy(startBar).and.callFake(mockStartBar)
  createFoo()
})

Changes

The first 2 commits does a bit of spec cleaning.

Then, we introduce mockable capabilities and use it in tests, mostly to replace all *Impl arguments and location mocking.

Please review commit by commit.

Test instructions

If tests passes, it should be good.

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

datadog-datadog-prod-us1 · 2026-01-28T13:46:49Z

⚠️ Tests

✨ Fix all issues with Cursor

⚠️ Warnings

❄️ 2 New flaky tests detected

cookie getCurrentSite caches the result from Safari 12.1.2 (Mac OS 10.14.6) (Datadog) (Fix with Cursor)
Expected spy cookie to have been called 2 times. It was called 0 times.
<Jasmine>
webpack:///packages/core/src/browser/cookie.spec.ts:46:43 <- /tmp/_karma_webpack_722687/commons.js:48873:49
<Jasmine>
cookie getCurrentSite returns the eTLD+1 for foo.bar.baz.example.com from Safari 12.1.2 (Mac OS 10.14.6) (Datadog) (Fix with Cursor)
Expected 'foo.bar.baz.example.com' to be 'example.com'.
<Jasmine>
webpack:///packages/core/src/browser/cookie.spec.ts:20:61 <- /tmp/_karma_webpack_722687/commons.js:48853:108
<Jasmine>

ℹ️ Info

🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 91.18%
• Overall Coverage: 77.27% (+0.01%)

View detailed report

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

🔗 Commit SHA: b98ddf1 | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

cit-pr-commenter-54b7da · 2026-01-28T13:55:03Z

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	169.44 KiB	169.55 KiB	+114 B	+0.07%	✅
Rum Profiler	4.31 KiB	4.31 KiB	0 B	0.00%	✅
Rum Recorder	24.54 KiB	24.54 KiB	0 B	0.00%	✅
Logs	56.72 KiB	56.62 KiB	-104 B	-0.18%	✅
Flagging	944 B	944 B	0 B	0.00%	✅
Rum Slim	126.26 KiB	126.16 KiB	-103 B	-0.08%	✅
Worker	23.63 KiB	23.63 KiB	0 B	0.00%	✅

🚀 CPU Performance

Pending...

🧠 Memory Performance

Pending...

🔗 RealWorld

This spec initialization is unnecessarily complex. This commit cleans that spec and moves one higher level assertion in `recorderApi.spec.ts`

A few test cases is logsPublicApi and preStartRum run test initialization multiple times (due to nested describe/beforeEach). This commit cleans this up

BenoitZugmeyer · 2026-02-12T09:11:02Z

/to-staging

gh-worker-devflow-routing-ef8351 · 2026-02-12T09:11:06Z

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2026-02-12 09:11:06 UTC ℹ️ Start processing command /to-staging

2026-02-12 09:11:12 UTC ℹ️ Branch Integration: starting soon, merge expected in approximately 14m (p90)

Commit 518ac81fe5 will soon be integrated into staging-07.

2026-02-12 09:22:05 UTC ℹ️ Branch Integration: this commit was successfully integrated

Commit 518ac81fe5 has been merged into staging-07 in merge commit 378f13e872.

Check out the triggered DDCI request.

If you need to revert this integration, you can use the following command: /code revert-integration -b staging-07

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packages/core/src/tools/mockable.ts

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Integrated commit sha: 518ac81 Co-authored-by: BenoitZugmeyer <benoit.zugmeyer@datadoghq.com>

thomas-lebeau · 2026-02-12T09:39:48Z

packages/logs/src/boot/logsPublicApi.ts

      strategy = createPostStartStrategy(initConfiguration, startLogsResult)
      return startLogsResult
-    },
-    startTelemetryImpl


👏 praise: ‏Nice! It was just argument drilling!

packages/rum-core/src/domain/resource/resourceCollection.ts

packages/rum/src/boot/recorderApi.ts

packages/rum/src/domain/deflate/deflateWorker.ts

thomas-lebeau · 2026-02-12T10:00:49Z

AGENTS.md

+### Unit Tests

- Spec files co-located with implementation: `feature.ts` → `feature.spec.ts`
+- Test framework: Jasmine + Karma. Spec files co-located with implementation: `feature.ts` → `feature.spec.ts`
+- Focus tests with `fit()` / `fdescribe()`, skip with `xit()` / `xdescribe()`
 - Use `registerCleanupTask()` for cleanup, NOT `afterEach()`
- Test framework: Jasmine + Karma
+- Mock values/functions: wrap with `mockable()` in source, use `replaceMockable()` or `replaceMockableWithSpy()` in tests (auto-cleanup)


👏 praise: ‏Yeah! 🤖

This change was generated by Claude to test coding agents capabilities to use the new mocking pattern. We used the following prompt: > Dear claude. In @packages/rum-core/src/domain/waitPageActivityEnd.ts and its spec, we have the `doWaitPageActivityEnd` function for testing purpose, because we don't want to run `createPageActivityObservable` in tests. Let's remove this extra function and use mocks instead instead.

BenoitZugmeyer · 2026-02-12T14:42:36Z

/to-staging

gh-worker-devflow-routing-ef8351 · 2026-02-12T14:42:40Z

View all feedbacks in Devflow UI.

2026-02-12 14:42:39 UTC ℹ️ Start processing command /to-staging

2026-02-12 14:42:49 UTC ℹ️ Branch Integration: starting soon, merge expected in approximately 14m (p90)

Commit b98ddf140b will soon be integrated into staging-07.

2026-02-12 14:53:42 UTC ℹ️ Branch Integration: this commit was successfully integrated

Commit b98ddf140b has been merged into staging-07 in merge commit 687815cd88.

Check out the triggered DDCI request.

If you need to revert this integration, you can use the following command: /code revert-integration -b staging-07

Integrated commit sha: b98ddf1 Co-authored-by: BenoitZugmeyer <benoit.zugmeyer@datadoghq.com>

BenoitZugmeyer force-pushed the benoit/mock branch from 55a8c24 to 7ab0afe Compare January 28, 2026 13:52

BenoitZugmeyer force-pushed the benoit/mock branch 3 times, most recently from 2fbac02 to 9182076 Compare January 29, 2026 20:46

BenoitZugmeyer added 4 commits February 11, 2026 14:38

✅ cleanup lazyLoadRecorder.spec.ts

574d5e9

This spec initialization is unnecessarily complex. This commit cleans that spec and moves one higher level assertion in `recorderApi.spec.ts`

✅ make sure we don't initialize things two times

f9cd842

A few test cases is logsPublicApi and preStartRum run test initialization multiple times (due to nested describe/beforeEach). This commit cleans this up

✅ introduce a new way to mock values

95f66d3

✅ replace all *Impl arguments with mockable

a339f5d

BenoitZugmeyer force-pushed the benoit/mock branch from c9c4f10 to a339f5d Compare February 11, 2026 14:21

BenoitZugmeyer changed the title ~~Benoit/mock~~ ✅ Introduce a new way to mock values in unit tests Feb 11, 2026

✅ use mockable to mock location

2f64528

BenoitZugmeyer force-pushed the benoit/mock branch from 1fe7847 to 518ac81 Compare February 11, 2026 14:54

BenoitZugmeyer marked this pull request as ready for review February 12, 2026 09:06

BenoitZugmeyer requested a review from a team as a code owner February 12, 2026 09:06

chatgpt-codex-connector bot reviewed Feb 12, 2026

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packages/core/src/tools/mockable.ts Outdated Show resolved Hide resolved

BenoitZugmeyer added 2 commits February 12, 2026 10:14

🤖 update AGENTS.md

16fd8e1

👌 apply codex suggestion

0ed26a5

good bot 🤖

BenoitZugmeyer force-pushed the benoit/mock branch from 518ac81 to 0ed26a5 Compare February 12, 2026 09:15

gh-worker-dd-mergequeue-cf854d bot added a commit that referenced this pull request Feb 12, 2026

Integrate benoit/mock (#4129) into staging-07

378f13e

Integrated commit sha: 518ac81 Co-authored-by: BenoitZugmeyer <benoit.zugmeyer@datadoghq.com>

gh-worker-dd-devflow-36fce6 bot added the staging-07 label Feb 12, 2026

BeltranBulbarellaDD approved these changes Feb 12, 2026

View reviewed changes

thomas-lebeau reviewed Feb 12, 2026

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BenoitZugmeyer added 2 commits February 12, 2026 14:07

👌 use mockable for taskQueue

0a1eb6d

👌 format nitpick

0ce309a

thomas-lebeau approved these changes Feb 12, 2026

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mormubis approved these changes Feb 12, 2026

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BenoitZugmeyer added 3 commits February 12, 2026 15:22

📝 adjust documentation

fcdf0fb

fix typecheck

b98ddf1

gh-worker-dd-mergequeue-cf854d bot added a commit that referenced this pull request Feb 12, 2026

Integrate benoit/mock (#4129) into staging-07

687815c

Integrated commit sha: b98ddf1 Co-authored-by: BenoitZugmeyer <benoit.zugmeyer@datadoghq.com>

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✅ Introduce a new way to mock values in unit tests#4129

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