tsconfig.json

{
  // https://github.com/microsoft/TypeScript/blob/1cd8e20f947513cc8c0c7c59e55b2f4524eff316/tests/baselines/reference/config/initTSConfig/Initialized%20TSConfig%20with%20advanced%20options/tsconfig.json#L7
  "compilerOptions": {
    "allowJs": true,
    //"checkJs": true, // Assumed from @ts-check.
    "target": "es2015", // but materials use ES2020 "??"
    "module": "commonjs",

    "strict": true,
    "noImplicitAny": true,
    "noUnusedLocals": false, // TODO enable when exports are added
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": false,

    "declaration": true, // Generate only declarations.
    //"declarationMap": true, // Emits .d.ts.map.
    "emitDeclarationOnly": true,
    "outDir": "types" // Emit .d.ts into types.
  },
  // Only include main sources.
  "include": ["ffl.js", "*Material.js"]
}