TypeScript’s utility types give you powerful ways to transform and manipulate existing types. If you’ve already used types like Partial or Pick, it’s time to expand your toolbox.
In this article, we'll focus on five highly useful utility types:
Omit<Type, Keys>Exclude<UnionType, ExcludedMembers>Extract<UnionType, IncludedMembers>NonNullable<Type>Parameters<Type>
1. Omit<Type, Keys>
Omit creates a new type by removing specified keys from another type.
Example
interface User {
id: number;
name: string;
email: string;
}
type UserWithoutEmail = Omit<User, 'email'>;
// Result:
type UserWithoutEmail = {
id: number;
name: string;
}
Use Case
Perfect when you need to reuse most of a type but exclude a few fields—for example, when hiding sensitive information.
2. Exclude<UnionType, ExcludedMembers>
Exclude removes members of a union that are assignable to another type.
Example
type Roles = 'admin' | 'user' | 'guest';
type NonGuest = Exclude<Roles, 'guest'>;
// Result: 'admin' | 'user'
Use Case
Useful when you want to filter out specific options from a union.
3. Extract<UnionType, IncludedMembers>
Extract is the opposite of Exclude. It keeps only the members that are assignable to another type.
Example
type Events = 'click' | 'hover' | 'scroll';
type UIEvents = Extract<Events, 'click' | 'hover'>;
// Result: 'click' | 'hover'
Use Case
Helpful when you want to narrow a union type to only valid choices.
4. NonNullable<Type>
NonNullable removes null and undefined from a type.
Example
type MaybeUser = User | null | undefined;
type DefinedUser = NonNullable<MaybeUser>;
// Result: User
Use Case
Great when you want to ensure a type is fully usable, for example, after a null-check.
5. Parameters<Type>
Parameters extracts the tuple of parameter types from a function type.
Example
function createUser(id: number, name: string): User {
return { id, name };
}
type Args = Parameters<typeof createUser>;
// Result: [id: number, name: string]
Use Case
Useful for reusing or forwarding function arguments dynamically.
Summary Table
| Utility Type | What It Does |
|---|
Omit<T, K> | Removes keys K from type T |
Exclude<T, U> | Removes members of union T that are assignable to U |
Extract<T, U> | Keeps only members of union T that are assignable to U |
NonNullable<T> | Removes null and undefined from type T |
Parameters<T> | Extracts argument types of function T as a tuple |
Wrapping Up
These utility types are essential when writing scalable TypeScript code. They allow you to:
- Create flexible APIs
- Simplify code reuse
- Ensure type safety in transformations
By mastering them, you'll write more expressive and maintainable code.