refactor: migrate polyfill module to TS

[jaissica12]

Background

• Continuing the effort to migrate remaining JavaScript modules in the mParticle Web SDK to TypeScript for improved type safety and developer experience. polyfill.js (Base64/UTF8 helpers and Array polyfills) was one of the remaining JS modules.

What Has Changed

• Converted src/polyfill.js to src/polyfill.ts with type annotations
• Consumers (mparticle-instance-manager.ts, persistence.js) continue to import ./polyfill unchanged
• No intentional runtime behavior, public API, or functional changes

Screenshots/Video

• N/A

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Additional Notes

• No dedicated unit/integration tests exist specifically for this module
• Mechanical JS → TS migration only

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes https://go/j/SDKE-1195

[cursor]

PR Summary

Low Risk
Type-only and style changes in a legacy polyfill module; consumers and public API are unchanged.

Overview
Renames polyfill.js to polyfill.ts and adds TypeScript types without changing runtime behavior or the default export shape.

Base64 and UTF8 helpers get explicit string parameters and return types; isArray is typed as (arg: unknown) => boolean. Internal var declarations become const / let where appropriate. Array polyfill methods (forEach, map, filter) keep the same signatures and logic—only typing and declaration style change.

Imports from ./polyfill in mparticle-instance-manager.ts and persistence.js stay the same.

^{Reviewed by Cursor Bugbot for commit 02a8a14. Bugbot is set up for automated code reviews on this repo. Configure here.}

[rmi22186]

The amount of changes to forEach, map, and filter is a little scary since there aren't unit tests for them. These were all originally copied and pasted from the links provided. Can you edit them to just make them do let/const, etc instead of doing a larger refactor?

[sonarqubecloud]

Quality Gate passed

Issues
29 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud