What does infer do in type ElementType = T extends (infer U)[] ? U : never?

Declares a type variable U that TS infers from matching the pattern

Asks the compiler for the array element type at runtime

Makes U an optional generic parameter defaulting to unknown

TypeScript · Middle · Question 7 of 75

What does `infer` do in `type ElementType<T> = T extends (infer U)[] ? U : never`?

✓Correct answer: Declares a type variable U that TS infers from matching the pattern
2Asks the compiler for the array element type at runtime
3Makes U an optional generic parameter defaulting to unknown

Explanation

infer is used only in the extends part of a conditional type: TS matches T against the pattern and "captures" the needed part. ElementType<string[]> → string. That's how ReturnType, Parameters and Awaited work.

What does `infer` do in `type ElementType<T> = T extends (infer U)[] ? U : never`?

Explanation

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What does infer do in type ElementType<T> = T extends (infer U)[] ? U : never?

Explanation

Further reading

Test yourself

What does `infer` do in `type ElementType<T> = T extends (infer U)[] ? U : never`?