TypeScript in this repo

New code is .ts, including tests, scripts, and Vite plugins. Do not add new .js. testem.cjs is the only CommonJS file.

npm run type-check (tsc --noEmit) and npm run lint both gate PRs via the Lint workflow. Run npm run type-check and npm run lint:fix after any edit under sources/, scripts/, vite/, or tests/ — all four are in tsconfig.json include. tsc is the typescript package, which this repo aliases to @typescript/typescript6. @typescript/native is also in devDependencies and is unused by workflows.

Erasable syntax only

Node runs first-party .ts by type-stripping it, with no tsx and no compile step. erasableSyntaxOnly is on, so no enums, no namespaces, no parameter properties. Anything that would need to emit runtime code is a type error.

Relative imports use the extension of the file on disk, so a .ts file importing a still-.js module writes .js:

import { markNonExecutableLinesInLcov } from "./mark-non-executable-lines.js";

allowImportingTsExtensions is on, so importing ./foo.ts is correct once the target is converted. Getting this backwards is the most common failure when converting a file — every importer has to move with it.

tsconfig deliberately relaxes three strictest flags

tsconfig.json extends @tsconfig/strictest and then turns off noUncheckedIndexedAccess, exactOptionalPropertyTypes, and noPropertyAccessFromIndexSignature, each with a comment explaining why. Do not re-enable them as a drive-by; pixel-index loops and metadata config bags depend on them being off.

Lint rules that bite

• Unused bindings that must exist need a _ prefix. argsIgnorePattern, varsIgnorePattern, and caughtErrorsIgnorePattern are all ^_.
• console.* is an error except console.error. Use console.error, or debugLog / debugWarn / debugGroup / debugGroupEnd / debugTable from sources/utils/debug.ts, which are gated on localhost or ?debug=.
• eslint.config.js applies typescript-eslint's recommended preset to **/*.ts only, and layers per-directory blocks for sources/, scripts/ + vite/, tests/, tests/node/, and tests/visual/. A converted file moves from the .js block to the .ts block and can pick up rules it was never subject to, so lint the file after renaming, not before.

Converting a .js file

Four non-test .js files remain:

• scripts/coverage/mark-non-executable-lines.js
• vite/vite-plugin-coverage-collect.js
• eslint.config.js
• playwright.config.js

The rest are under tests/. Ordinary *_spec.js files are not named by hardcoded path. These harness files are:

• tests/tests.js — named in tests_run.html
• tests/bdd-globals.js — the mocha-globals alias target in vite.config.ts, in two HTML import maps (scripts/zip/zip-export-profile-runner.html, tests/fixtures/issue-382/issue382-golden-runner.html), and asserted by path in tests/node/scripts/generateSources/vite_config_factory_and_resolve_spec.ts
• tests/node/run-node-tests.js — named in testem.cjs before_tests
• tests/vitest-setup.js — imported by tests/tests.js
• tests/testem-firefox-user.js — named in testem.cjs

Before renaming anything, grep for the filename. Breaking a harness path fails a command rather than the type-check:

• scripts/coverage/mark-non-executable-lines.js — named in package.json test:node:coverage, and imported by merge-browser-coverage.ts.
• vite/vite-plugin-coverage-collect.js — imported by vite.config.ts.

Checklist for a conversion:

1. git mv the file, then update every importer to the new extension.
2. Update hardcoded paths in package.json, testem.cjs, HTML files, and any spec that asserts on the path.
3. npm run type-check — the file is newly checked, so expect real errors.
4. npm run lint:fix — it may have moved into a stricter ESLint block.
5. Run whatever command names the file, not just the test suite. A broken before_tests path or import map does not surface as a test failure.
6. Check codecov.yml ignore:. If the file is not ignored, it now owes covered lines: coverage.

Adding a .ts browser spec

Canvas already has several TypeScript specs (renderer_spec.ts, renderer-composite_spec.ts, load-images_spec.ts, and the palette-recolor-*_spec.ts files). Leftover *_spec.js files under tests/ stay JS until converted. Two things differ from those leftover .js specs:

• Register it with its real extension, since relative imports use the on-disk extension:

  import "./canvas/renderer-composite_spec.ts";
  

• It is type-checked under @tsconfig/strictest as soon as it exists, which leftover .js specs are not (checkJs is off). Run npm run type-check; annotate DOM hosts, sinon sandboxes, and catalog handles the way renderer-composite_spec.ts already does.

Everything else about writing the spec is unchanged: write-spec.