Track TypeScript 7 migration readiness

[kvithayathil]

Summary

Track the conditions and verification work required before VoteCatcher can move its frontend from TypeScript 6 to TypeScript 7.

TypeScript 7.0 is a native compiler with substantial performance gains, but it has no stable programmatic API. Svelte tooling embeds that API, so a direct upgrade currently breaks the project type-check workflow. Do not force peer-dependency overrides or replace bun run check while this issue is open.

Current State

• Frontend uses TypeScript 6.0.3, SvelteKit, svelte-check, typescript-eslint, and Storybook Svelte.
• bun run check is the authoritative full type check because it validates .svelte files.
• TypeScript 7.0.2 CLI can check the TS/JS subset, but native tsc omits .svelte source files; it cannot be a merge gate.
• Svelte language-tools issue #3063 remains open. Maintainers confirm TS 7 is fundamentally incompatible with svelte-check and svelte-package today.

Subtasks

Upstream Readiness

• Monitor sveltejs/language-tools#3063 until maintainers document end-to-end TypeScript 7 support.
• Monitor sveltejs/language-tools#3073; do not treat alias detection or a clearer runtime error as full TS 7 support.
• Confirm a released SvelteKit version accepts TypeScript 7 in its peer dependency range.
• Confirm a released typescript-eslint version accepts TypeScript 7; upgrade parser, plugin, and meta package together.
• Confirm Svelte Storybook and its svelte2tsx integration support TypeScript 7, or document evidence that Storybook is isolated from the application compiler.
• Confirm TypeScript 7.1 or later documents the stable programmatic API required by embedded-language tooling.

Isolated Compatibility Upgrade

• Create a dedicated migration branch/worktree after every upstream readiness condition is met.
• Upgrade TypeScript and all dependent frontend tooling to mutually compatible released versions. Do not use peer-dependency overrides.
• Run bun install --frozen-lockfile after lockfile regeneration to prove deterministic resolution.
• Review tsconfig.json and generated SvelteKit config for TS 7 defaults and hard errors: rootDir, explicit types, moduleResolution, removed legacy module targets, baseUrl, and import attributes.
• Audit JS/JSDoc files because VoteCatcher enables allowJs and checkJs; TS 7 changes several JSDoc/Closure-style interpretations.
• Review type-level string utilities for the TS 7 Unicode code-point template-literal behavior.

Stability And Compatibility Gates

• bun run check reports no new errors and continues to check .svelte components.
• bun run lint passes.
• bun run test:unit passes.
• bun run test:e2e passes.
• bun run build passes.
• bun run build-storybook passes.
• Run the repository CI-equivalent command and compare diagnostics with the TS 6 baseline.
• Verify local editor diagnostics, completions, go-to-definition, and rename in .svelte, .ts, and .js files. Until Svelte supports the native TS language server, test with the TypeScript 7 VS Code language server disabled.
• Measure clean-cache and warm-cache check times and memory against the TS 6 baseline; record the machine/runner configuration and command lines.

Rollout

• Add the smallest necessary script/documentation changes for the final supported workflow.
• Update CI only after all preceding gates pass.
• Define rollback as reverting the dependency and lockfile commit to the prior TS 6 toolchain.

Acceptance Criteria

• Every upstream readiness item is supported by a linked release note, peer range, or maintainer statement.
• The upgrade branch passes all stability and compatibility gates without peer overrides.
• The resulting TypeScript configuration has no TS 7 deprecation/hard-error regressions.
• svelte-check remains functional and authoritative for Svelte component diagnostics.
• The issue records benchmark and editor verification results before the migration PR is opened.

References

• TypeScript 7.0 announcement - official release notes: native compiler, lack of 7.0 programmatic API, side-by-side TS 6/7 guidance, TS 6-to-7 defaults and breaking changes, embedded-language limitations.
• TypeScript compiler options reference - authoritative compiler-option documentation for migration configuration review.
• TypeScript 7.0 changes - documented behavior differences, including JavaScript analysis.
• Svelte language-tools issue #3063 - confirmed TS 7 incompatibility.
• Svelte language-tools PR #3073 - in-progress alias/error-handling work, not complete embedded TS 7 support.