TS2312

Interface cannot extend a union

An interface can only extend an object type or intersection of object types with statically known members.

The compiler’s own words. Not translated — this is the string you pasted into a search box.

An interface tried to inherit from a union whose members are not all known at once.

Reproduction

type Choice = { a: 1 } | { b: 2 }

interface Both extends Choice {
  c: 3
}

The build asserts this emits exactly this code.

Why the compiler says this

Interface inheritance merges a definite set of object members. A union means one branch or another, so there is no single member list for the interface declaration to inherit and merge.

Fixes

type Choice = { a: 1 } | { b: 2 }

type WithC = Choice & { c: 3 }

Use an intersection alias to add members to every branch of a union.

interface Base {
  a: number
}

interface Extended extends Base {
  c: number
}

Extend an interface or object type when the base really has a fixed shape.

Takeaway

Interfaces extend known object shapes. Use type aliases to compose unions and intersections.

Where to go next

Type aliases Try it in the Lab

Errors