Logic with conditional

Learn to build type-level logic with conditional types.

Logic with conditionals

With conditional types you can build type-level logic. The pattern T extends U ? never : T removes from T the members assignable to U; this is the basis of Exclude. Conversely T extends U ? T : never keeps only those assignable to U; this is the basis of Extract.

type Cikar<T, U> =
  T extends U ? never : T;
type R = Cikar<"a" | "b", "b">;
// "a"

never disappears from the union

With distributive conditionals, never disappears from a union. "a" | never | "c" = "a" | "c". This makes filtering logic work naturally: members that fail the condition become never and drop out of the result.

Exercise Remove from a union

Write the conditional type Cikar<T, U> as T extends U ? never : T. Define type Kalan as Cikar<"a" | "b" | "c", "b">. Define variable dizi with type Kalan[] and assign ["a", "c"]. Print dizi[0] and dizi[1] on separate lines.

Show example solution

type Cikar<T, U> = T extends U ? never : T;
type Kalan = Cikar<"a" | "b" | "c", "b">;
const dizi: Kalan[] = ["a", "c"];
console.log(dizi[0]);
console.log(dizi[1]);

Test yourself

What does Cikar<"a" | "b" | "c", "b"> produce?
```
type Cikar<T, U> = T extends U ? never : T;
type R = Cikar<"a" | "b" | "c", "b">;
```
- "b"
- "a" | "c"
- "a" | "b" | "c"
- never
Show answer

"a" | "c" — Distribution evaluates each member separately: "a" and "c" are not assignable to U so they stay; "b" is assignable so it becomes never and drops from the union. The result is "a" | "c".

Use never for filtering

type Cikar<T, U> =
  T extends U ? ____ : T;
type R = Cikar<"a" | "b", "b">;
// R: "a"

Show answer

type Cikar<T, U> =
  T extends U ? never : T;
type R = Cikar<"a" | "b", "b">;
// R: "a"

Logic patterns

T extends U ? never : T

Removes those assignable to U
T extends U ? T : never

Keeps those assignable to U
never

Disappears from the union
"a" | never | "c"

Equals "a" | "c"

Next lesson: Summary - Conditional types

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