No. 220 · July 28, 2026 · Moderate

Rebuild Exclude

Implement MyExclude<T, U> so it removes from the union T every member assignable to U. It is one line — the subtlety is why that line works at all.

Try the puzzle yourself

Par 3

Puzzle

rebuild-exclude.ts

type MyExclude<T, U> = ???

type MyExclude<T, U> = ???

Stroke 1 of 3Not run yet

Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

Checks

MyExclude<'a' | 'b' | 'c', 'a'>—
→ 'b' | 'c'
MyExclude<string | number, number>—
→ string
MyExclude<'a', 'a'>—
→ never
MyExclude<'a' | 'b', 'c'>—
→ 'a' | 'b'

How a check is judged Exact type equality, not assignability — an intersection is not the same as the flattened object.

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Annotated solution

Published July 29, 2026

The solution

type MyExclude<T, U> = T extends U ? never : T

The common wrong answer

type MyExclude<T, U> = [T] extends [U] ? never : T

Wrapping T in a tuple switches distribution off, so the whole union is compared at once. ["a" | "b" | "c"] extends ["a"] is false, and the entire union comes back unfiltered.

Line by line

T extends U
T here is *naked* — it appears alone on the checked side, not wrapped in a tuple, array or object. That is the exact condition that triggers distribution over a union.
? never : T
Each member either becomes never or survives as itself. Unioning the results drops the nevers automatically, because never is the identity element of union.

Takeaway

Distribution is the difference between asking a question about a union and asking it about each member. [T] extends [U] is the switch that turns it off.

Uses

Conditional types Exclude Awaited Conditional types Make a switch exhaustive Model states that narrow TS1110 TS2367 TS2532 TS18047