Unions

Extract

Keeps from a union only the members assignable to `U`. The complement of `Exclude`.

What it is

The same distributive conditional with the branches swapped. Its most useful application is not filtering primitives but re-narrowing a discriminated union: given a union of shapes and a discriminant, `Extract` gets you the one member you meant.

Examples

```
Extract<'a' | 'b' | 'c', 'a' | 'c'>
```
→"a" | "c"

Extract<string | number | boolean, string | boolean>

→string | boolean

```
Extract<Shape, { kind: 'circle' }>
```
→{ kind: "circle"; r: number; }
This is the one that earns its keep: pulling a single member back out of a discriminated union by its tag.

Each resolved type above was printed by TypeScript 5.9.3, not written by hand.

What it does not do

It does not narrow a value. `Extract` operates on types; narrowing a variable at run time still needs a check the compiler can follow.
It does not error when nothing matches — it gives you `never`, quietly, and the mistake surfaces somewhere else.

Takeaway

`Extract<Union, { tag: "x" }>` is the type-level version of the narrowing you already write with `if`. Reach for it when a generic needs one member of a union it was handed.