What does infer N extends number do in a conditional type? type ToNum = S extends `${infer N extends number}` ? N : never; type X = ToNum ;

Infers N with a constraint - X becomes the literal type 42

Converts the string '42' to a number at runtime via Number()

Causes a compile error: infer doesn't support extends

TypeScript · Senior · Question 13 of 75

What does `infer N extends number` do in a conditional type?

type ToNum<S> = S extends `${infer N extends number}` ? N : never;
type X = ToNum<'42'>;

✓Correct answer: Infers N with a constraint - X becomes the literal type 42
2Converts the string '42' to a number at runtime via Number()
3Causes a compile error: infer doesn't support extends

Explanation

A constraint on infer (TS 4.7+) not only checks compatibility but also lets you infer a literal numeric type from a string. Previously this required nested conditional types.

What does `infer N extends number` do in a conditional type?

Explanation

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