TypeScript · Senior · Question 4 of 75

How do you turn off a conditional type's distribution over a union?

✓Correct answer: Wrap both sides in a tuple: [T] extends [U] ? X : Y
2Add the distributive: false option to compilerOptions
3Use infer U instead of extends in the check

Explanation

Distribution happens only when the checked type is a "naked" parameter. The tuple [T] makes it composite, so the union is checked as a whole: [string | number] extends [string] → false. There's no compiler setting for this.

Further reading

TS Handbook: Conditional Types ↗

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