TypeScript project configuration strict mode and performance tips

What strict mode enables

"strict": true enables several flags at once. The most impactful:

{
  "compilerOptions": {
    "strict": true,
    // Included: strictNullChecks, noImplicitAny,
    // strictFunctionTypes, strictPropertyInitialization,
    // strictBindCallApply, noImplicitThis
  }
}

Additional recommended flags

{
  "compilerOptions": {
    "noUncheckedIndexedAccess": true,  // arr[i] returns T | undefined
    "exactOptionalPropertyTypes": true, // ? means absent, not undefined
    "noImplicitReturns": true,          // all paths must return
    "noFallthroughCasesInSwitch": true
  }
}

Performance: large codebases

{
  "compilerOptions": {
    "incremental": true,      // cache build info
    "skipLibCheck": true,     // skip .d.ts checking in node_modules
    "isolatedModules": true   // compatible with esbuild/swc
  }
}

Path aliases clean up deep imports:

{
  "compilerOptions": {
    "paths": { "@/*": ["./src/*"] }
  }
}

Enable noUncheckedIndexedAccess from project start — retrofitting it later is painful but it catches real bugs on every array access.