TypeScript strict mode

Why strict?

"strict": true in tsconfig.json enables eight sub-flags: noImplicitAny, strictNullChecks, strictFunctionTypes, strictBindCallApply, strictPropertyInitialization, noImplicitThis, useUnknownInCatchVariables, and alwaysStrict. Together they eliminate entire classes of bugs that slip through loose TypeScript configs.

The config

{
  "compilerOptions": {
    "strict": true,
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "skipLibCheck": true
  }
}

What each flag catches

• noImplicitAny — variables and parameters must have explicit types
• strictNullChecks — null and undefined are not assignable to every type
• strictFunctionTypes — function parameter bivariance is checked correctly
• strictPropertyInitialization — class properties must be set in constructor or have definite assignment

Adopting incrementally

If your codebase is large, toggle strict on one directory at a time using project references or separate tsconfig files. Run npx tsc --noEmit after each change. Fix errors from the bottom of the list up — later errors are often cascading from earlier ones.