TypeScript Conditional Types

Conditional types allow you to describe relations between input and output types using logic similar to ternary expressions.

Basic Syntax

SomeType extends OtherType ? TrueType : FalseType;

If SomeType is assignable to OtherType, the result is TrueType.

Constraints and Extraction

Conditional types can be used to extract information from a type when a constraint is met.

type MessageOf<T> = T extends { message: unknown } ? T["message"] : never;

The infer Keyword

Used to declaratively introduce a new generic type variable within the extends clause of a conditional type.

type GetReturnType<T> = T extends (...args: any[]) => infer R ? R : any;

Distributive Conditional Types

When a conditional type acts on a generic type and is given a union, it "distributes" across the union members.

• ToArray<string | number> becomes ToArray<string> | ToArray<number>.
• Disabling Distributivity: Wrap the types in square brackets (e.g., [T] extends [any]).

References

• Source: 00_Raw/typescript-handbook.md
• typescript-moc
• typescript-generics
• typescript-type-operators