02
Annotated solution
Published August 31, 2026The solution
type OmitByType<T, U> = { [K in keyof T as T[K] extends U ? never : K]: T[K] }
The common wrong answer
type OmitByType<T, U> = { [K in keyof T]: T[K] extends U ? never : T[K] }
This filters the value and leaves the key standing. `a` is still there, now typed `never` — a property nobody can ever supply a value for, which is not the same thing as a property that does not exist. Filtering on the value side can empty a key; it cannot delete one.
Line by line
as T[K] extends U ? never : KThe `as` clause renames the key being produced. Renaming it to `never` is the one way to make a mapped type emit no property at all for that iteration.
OmitByType<{ a: 1; b: 2 }, never>Nothing is dropped here, because `1 extends never` is false. `never` is the empty set: no type is assignable to it, so no key matches and everything survives.
Takeaway
Keys are removed in the key clause, never in the value clause. If you find yourself producing `never` as a property type, you almost certainly meant to produce no property.