[no-Jira] Migrate to TypeScript 6

[canac]

Description

Upgrade to TypeScript 6.0

Testing

• Test that CSV upload and profile image upload still works

Checklist:

• I have given my PR a title with the format "MPDX-(JIRA#) (summary sentence max 80 chars)"
• I have applied the appropriate labels (Add the label "Preview" to automatically create a preview environment)
• I have run the Claude Code /pr-review command locally and fixed any relevant suggestions
• I have requested a review from another person on the project
• I have tested my changes in preview or in staging
• I have cleaned up my commit history

[github-actions]

Preview branch generated at https://typescript-6.d3dytjb8adxkk5.amplifyapp.com

[github-actions]

Bundle sizes [mpdx-react]

Compared against 1d25db1

No significant changes found

[canac]

Multi-Agent Code Review

Verdict: APPROVED WITH SUGGESTIONS — no blockers, mechanical TS 5.6 → 6.0 bump. 1 Important + 4 Medium/Suggestion findings posted as inline comments.

Risk Assessment

• Score: 10/10 (capped) — CRITICAL
• Driven by typescript compiler-version blast radius across the whole repo, not by semantic risk in the diffs themselves. The actual changes are minimal and mechanical.
• Gate merge on a green whole-repo yarn lint:ts + yarn test CI run, plus a manual smoke test of the two upload paths (CSV import + avatar upload) given the new Uint8Array(...) wrap.

Agents Run

Security, Architecture, Testing, UX, Standards, Financial Reporting (Data Integrity skipped — no schema/migration/Apollo cache/mutation/pagination changes).

Key Findings

1. [Important 7.0] Root cause of every String(t(...)) wrap is stale i18next@21.9.2 / react-i18next@11.18.6. Newer versions return string by default, or augment via interface CustomTypeOptions { returnNull: false }. Eliminates the churn and de-risks the ~73 other unwrapped aria-label={t(...)} sites that compile today only because looser MUI prop typings tolerate the wider return type. Follow-up ticket recommended — not blocking.

2. [Medium 6.5] Inconsistent fix style: ExpectedMonthlyTotalReportTable.tsx hoists const label = t(...) while the other 7 files inline String(t(...)). Pick one. Flagged by 4 agents (Architecture, Testing, UX, Standards).

3. [Medium 6.0] New tsconfig.json types: ["node", "jest", "@testing-library/jest-dom"] narrowing is load-bearing on a transitive /// <reference types="google.maps" /> from @googlemaps/js-api-loader. Add an explicit reference in app.d.ts so a future dep upgrade can't silently drop ambient types.

4. [Medium 5.0] Selective application — only ~7 of ~80 aria-label={t(...)} sites were wrapped (rest compile via looser MUI prop typings). Confirm yarn lint:ts passes whole-repo, not just changed files, before merging.

5. [Suggestion 4.0] @types/testing-library__jest-dom is now redundant — the runtime @testing-library/jest-dom package ships its own types (and is referenced explicitly in the new types array). Remove from devDependencies.

6. [Suggestion 4.0] No tests cover the two upload endpoints (pre-existing, excluded by jest coverage policy). new Uint8Array(Buffer) wrap is byte-identical at runtime, but smoke-test both upload paths before merge.

What's Clean

• Security: No findings. NextAuthOptions['providers'] swap is type-equivalent to Provider[]. new Uint8Array(buffer) copies bytes 1:1 (no risk of reading past buffer). typescript correctly placed in devDependencies. No NextAuth callback / CSP / env-var / redirect changes.
• Financial Reporting: No calculation, aggregation, currency, or date logic touched.
• Standards Checklist: All applicable items PASS. No new any, no @ts-ignore, no debug output, no commented-out code, no new Date().
• Architecture positives: moduleResolution: "bundler" is the correct choice for Next 15 + TS 6. Removing baseUrl while adding explicit paths is the right move. declare module '*.css' confirmed needed (2 real CSS imports in _app.page.tsx).

Dependency Impact

Global: tsconfig.json, app.d.ts, package.json, .yarn/sdks/typescript/package.json. No breaking runtime changes.

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Findings on Related Files (Not in Diff Hunk)

The following finding is on a file in this PR but on a line outside the diff hunk, so it cannot be posted as a line-anchored comment:

[Suggestion 4.0] package.json:116 — @types/testing-library__jest-dom is now redundant. The runtime @testing-library/jest-dom package ships its own types and is referenced explicitly in the new tsconfig.json types array. Remove this DefinitelyTyped package from devDependencies: yarn remove @types/testing-library__jest-dom. Informational — not blocking.

[canac]

[Medium] With the new `tsconfig.json` `types: ["node", "jest", "@testing-library/jest-dom"]` narrowing, ambient `google.maps` types resolve only because `@googlemaps/js-api-loader` carries a transitive `/// `. Any future upgrade of that package that drops the reference will silently lose those ambient types. Recommend adding an explicit reference here so the chain isn't load-bearing on a transitive dep's internals:

/// <reference types="google.maps" />
declare module '*.graphql' { ... }
declare module '*.css';
declare module '*.svg';

[canac]

@wjames111 This is not urgent at all. But I remembered I had this working in a branch a few weeks ago, so I might as well push it up. This should take care of all the breaking changes so that when TypeScript 7.0 comes out (which is supposed to be ~10x faster 🚀 ) it will be an easy upgrade.

[wjames111]

Looks great, looking forward to the upgrade! Claude mentioned this as well, "the String(t(...)) workaround papered over a typing problem that's better solved with one module-augmentation block in app.d.ts" if this is possible to do a in a module wouldn't it be better just to do it there?