object-transforms

`Exclude<T, U>`

Filters a union T by removing every member that is assignable to U. Distributes over union members via the conditional type, producing the set-difference T \ U.

Definition

type Exclude<T, U> = T extends U ? never : T

Usage patterns

Pattern	Purpose
Exclude<T, null \| undefined>	Equivalent to NonNullable<T>
Exclude<keyof T, K>	Compute complementary key set (basis of Omit)
Exclude<T, Function>	Remove function members from a mixed union
Exclude<T, never>	Identity — returns T unchanged

Examples

type Fruit = 'apple' | 'banana' | 'cherry';
type NotBanana = Exclude<Fruit, 'banana'>;

Result: 'apple' | 'cherry'

type T = string | number | boolean;
type NonBool = Exclude<T, boolean>;

Result: string | number

type Keys = Exclude<keyof User, 'password'>;

Internal building block of Omit — enumerates safe keys

type X = Exclude<'a' | 'b', 'a' | 'b' | 'c'>;

Result: never — every member of T is assignable to U

Gotcha

Exclude only works on union types — a plain object type passed as T will not be reduced. The distributive behavior can also over-fire on generics; wrap the parameter in a tuple `[T] extends [U]` to disable distribution.

Related utility types

Extract<T, U>

Keeps only those members of union T that are assignable to U. The dual of Exclude, giving the set-intersection T ∩ U.

NonNullable<T>

Removes null and undefined from the type T. In TS 4.9+ it is implemented as `T & {}`, which drops nullish members without altering the rest.

Omit<T, K>

Constructs a type by removing the properties in K from T. Unlike Pick, its K is not constrained to keyof T, so extra keys are silently ignored.

infer R

A keyword usable only inside the extends clause of a conditional type that introduces a fresh type variable inferred from the matched shape. It is the primitive that powers ReturnType, Parameters, InstanceType, Awaited, and countless custom utilities.

Partial<T>

Constructs a type where every property of T is set to optional. Useful for representing update payloads, patch objects, and configuration overrides where all fields may be omitted.

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