chore: convert lib/utils.js to ESM

[arun-357]

PR Checklist

• Addresses an existing open issue: fixes 🛠️ Repo: Convert lib/utils.js to ESM #5964
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Part of #5400. Renames lib/utils.js → lib/utils.mjs and converts it to ESM named exports, updating every runtime call site and test in the same change. No behavior change.

• Each exports.X = … becomes an inline export function/export const; internal exports.X self-references are now direct calls. No default/aggregate export (per AGENTS.md).
• Call sites updated to match: CJS consumers require("./utils.mjs"), ESM named-import consumers from "./utils.mjs", and namespace consumers switched from import utils from to import * as utils from (a default import of a named-export module is undefined).
• Tests: sinon cannot stub ES modules, so the sinon.stub(utils, …) calls were reworked — the unasserted isString/noop stubs were removed, and the isBrowser stubs were replaced with process.browser toggling (isBrowser() is Boolean(process.browser), so the same code path and assertions run). rewiremock keys/import paths were repointed to .mjs.

Verified: test-node suites, test-smoke, eslint, tsc, and a clean+build browser bundle all pass; lib/utils.js was not listed in package.json's browser map, so no config change was needed.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: arun-357 / name: arun-357 (cce77da)

[codecov]

Codecov Report

❌ Patch coverage is 19.04762% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.43%. Comparing base (bab1d4e) to head (c4fd7a7).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/utils.js	0.00%	34 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6071      +/-   ##
==========================================
- Coverage   82.25%   76.43%   -5.83%     
==========================================
  Files          61       61              
  Lines        4564     4561       -3     
  Branches      989      985       -4     
==========================================
- Hits         3754     3486     -268     
- Misses        810     1075     +265     

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[arun-357]

For code coverage, its not able to pick the natively loaded .mjs but tests still asserts the full utils as before.

[arun-357]

Under the new type: module, plain .js is ESM — so do we want to keep it as .mjs or .js ? @mark-wiemer

[JoshuaKGoldberg]

@arun-357 .js

[arun-357]

@JoshuaKGoldberg No issue then, it is already a .js file in this PR.

stale

[arun-357]

@mark-wiemer I fixed the conflicts.