♻️ move pure utilities to @datadog/js-core/util

[BenoitZugmeyer]

Motivation

Move more utilities to js-core. The goal is to eventually move batch into js-core, and this is a step toward it. All those utilities are dependencies of batch. It's not the only ones, but this PR is large enough.

Changes

• byteUtils (computeBytesCount, concatBuffers, ONE_KIBI_BYTE, ONE_MEBI_BYTE, Uint8ArrayBuffer) moved to @datadog/js-core/util; spec moved alongside.
• Context types (Context, ContextValue, ContextArray) moved to @datadog/js-core/util.
• mockable (mockable, mockableReplacements) moved to @datadog/js-core/util.
• jsonStringify (jsonStringify, detachToJsonMethod, ObjectWithToJsonMethod) moved to @datadog/js-core/util; spec moved alongside. The noop dependency is inlined to avoid pulling browser-specific utilities into js-core.
• Uint8ArrayBuffer simplified from a custom interface to Uint8Array<ArrayBuffer>.
• All consumers (browser-core internals, browser-logs, browser-rum, browser-rum-core, browser-rum-slim, browser-worker, browser-debugger, developer-extension, e2e tests) now import directly from @datadog/js-core/util instead of going through browser-core re-exports.
• @datadog/js-core added as a dependency of @datadog/browser-worker.

Test instructions

This is a pure refactor with no observable behaviour change. All existing unit tests cover the moved code. To sanity-check the SDK still initialises and sends data correctly:

1. Run yarn dev and open http://localhost:8080.
2. Check the Network tab — RUM and Logs requests should still be sent to the intake endpoints on the configured interval.
3. Navigate away from the page and confirm a flush request is sent on beforeunload/visibilitychange.

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-us1-2]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 77.01% (+0.00%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 3b565c9 | Docs | View more details | Give us feedback!}

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	182.02 KiB	182.02 KiB	-3 B	-0.00%	✅
Rum Profiler	8.43 KiB	8.43 KiB	0 B	0.00%	✅
Rum Recorder	25.32 KiB	25.32 KiB	0 B	0.00%	✅
Logs	57.96 KiB	57.98 KiB	+16 B	+0.03%	✅
Rum Salesforce	N/A	140.10 KiB	N/A	N/A	N/A
Rum Slim	140.08 KiB	140.09 KiB	+15 B	+0.01%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	206.82 KiB	N/A	N/A	N/A
Rum-shopify Profiler	N/A	8.43 KiB	N/A	N/A	N/A
Rum-shopify Recorder	N/A	3.74 KiB	N/A	N/A	N/A

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-04T13:40:02.151714Z	3b565c9	New commits

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[chatgpt-codex-connector]

💡 Codex Review

browser-sdk/packages/js-core/src/util/jsonStringify.ts

Line 22 in 5b800e9

const restoreValuePrototypeToJson = detachToJsonMethod(Object.getPrototypeOf(value))

Handle objects without a prototype before detaching toJSON

issue: When the new public jsonStringify API receives Object.create(null), Object.getPrototypeOf(value) returns null, which is passed to detachToJsonMethod and dereferenced before the try block. This throws even though native JSON.stringify accepts null-prototype objects and the function accepts unknown; guard the missing prototype or include this setup in the protected path.

AGENTS.md reference: packages/js-core/AGENTS.md:L33-L44

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[BenoitZugmeyer]

🤖 Regarding the P2 Object.create(null) note on jsonStringify: the null-prototype behavior predates this PR — the function was moved verbatim from browser-core. I'll address it in a follow-up rather than scope-creep this move.

[chatgpt-codex-connector]

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Reviewed commit: e91981b3c3

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