tsconfig Strictness

The strict Flag and Its Family

`strict: true` enables `strictNullChecks`, `strictFunctionTypes`, `strictBindCallApply`, `strictPropertyInitialization`, `noImplicitAny`, `noImplicitThis`, and `alwaysStrict`. New projects should start with strict enabled from day one.

Migrating legacy JavaScript enables strict flags incrementally — turn on one flag per PR, fix violations, and prevent regression with CI. `strictNullChecks` alone catches the most production bugs.

• `strict: true` enables the full suite
• Enable flags incrementally in migrations
• strictNullChecks has highest bug-finding ROI

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true
  }
}

High-Value Additional Flags

`noUncheckedIndexedAccess` adds `undefined` to indexed access — arrays and records require explicit checks. `exactOptionalPropertyTypes` distinguishes missing vs explicitly undefined optional props. `noImplicitReturns` ensures all code paths return in functions with return types.

`verbatimModuleSyntax` enforces explicit `import type` and prevents accidental value imports of types. Enable when tooling supports it for cleaner output and fewer circular import bugs.

• noUncheckedIndexedAccess for array safety
• exactOptionalPropertyTypes for precise optionals
• verbatimModuleSyntax for clean imports

Module and Interop Settings

`esModuleInterop` and `allowSyntheticDefaultImports` simplify default imports from CommonJS modules. `isolatedModules` ensures each file transpiles independently — required for Babel and esbuild and catches re-export issues.

`skipLibCheck` skips typechecking of declaration files — faster builds with slightly less safety. Most projects enable it; fix upstream @types issues locally with module augmentation when needed.

• isolatedModules for Babel/esbuild
• esModuleInterop for CJS default imports
• skipLibCheck for build speed

Project References and Monorepos

Split monorepos with `composite: true` and project references for incremental builds. Each package has its own tsconfig extending a base config with shared strict flags.

Path mapping at the root coordinates packages: `"references": [{ "path": "./packages/ui" }]`. `tsc --build` compiles the dependency graph in order, caching outputs.

• Project references for monorepos
• Shared base tsconfig for consistency
• Incremental builds with composite

Balancing Strictness and Velocity

Stricter configs slow initial migration but accelerate long-term development. Document any intentional `@ts-expect-error` with ticket links and expiration plans — suppressions without context become permanent holes.

Use ESLint TypeScript rules (`@typescript-eslint`) alongside compiler flags for patterns the compiler cannot enforce — consistent type imports, explicit return types on exports, and ban on non-null assertions in critical paths.

• Document ts-expect-error with tickets
• ESLint complements compiler flags
• Stricter configs pay off over time