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TypeScript·Advanced·Coding·1 min read

What are conditional types?

Short interview answer

A conditional type selects one type or another based on assignability. When the checked type is a naked type parameter, it distributes over unions. Wrapping both sides in tuples can suppress distribution.

Example

TypeScript
type Unwrap<T> = T extends Promise<infer U> ? U : T;type A = Unwrap<Promise<string>>; // stringtype B = Unwrap<number>;          // number
type NonNull<T> = T extends null | undefined ? never : T;type C = NonNull<string | null>; // string  (distributes over the union)

Key takeaway

Explain the underlying mental model clearly, then support it with a concrete example and its trade-offs.

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