TypeScript Strict Mode: Best Practices

Why Strict Mode?

TypeScript's strict flag is a shorthand that enables a collection of checks that prevent common runtime errors. Projects that skip it often accumulate subtle null-reference and type-widening bugs.

Add it to tsconfig.json:

{
  "compilerOptions": {
    "strict": true
  }
}

What Gets Enabled

strict turns on:

• strictNullChecks — null and undefined are not assignable to other types
• noImplicitAny — variables without an inferred type must be explicitly typed
• strictFunctionTypes — stricter checking on function parameter types
• strictPropertyInitialization — class properties must be initialized in the constructor

Patterns That Help

Prefer unknown over any

function parseJson(input: string): unknown {
  return JSON.parse(input)
}

Force callers to narrow the type before use.

Use Discriminated Unions

type Result<T> =
  | { status: 'ok'; data: T }
  | { status: 'error'; message: string }

Exhaustive switch statements on status will catch missing cases at compile time.

Type-Only Imports

import type { User } from './types'

Prevents accidental runtime imports of type-only modules and makes bundler tree-shaking more reliable.

Incremental Adoption

For existing codebases, enable options one at a time, starting with strictNullChecks. Fix the errors in each batch before moving to the next flag.

Strict mode TypeScript is one of the highest-value investments in a codebase's long-term health.