Type and interface inheritance

TypeScriptMiddle~1 min read

Not directly. Types (type) do not support the extends keyword at creation, that is, you cannot write this:

javascript

interface A {
  name: string;
}

type B extends A = { age: number }; // Error

But you can combine a type and an interface through an intersection (&), which is essentially equivalent to inheritance:

javascript

interface A {
  name: string;
}

type B = A & { age: number }; // Works

So a type does not "inherit" an interface, but it can extend it using &.

Yes, if the type is an object structure.

TypeScript lets an interface inherit from a type through extends, but only if the type describes an object (not a primitive, a union, etc.):

javascript

type A = { name: string };

interface B extends A {
  age: number;
}

const obj: B = { name: "Tim", age: 25 }; // Works

But if the type is not an object (for example, a union or a primitive), there will be an error:

javascript

type C = string | number;

interface D extends C {} // Error: an interface can only inherit object types

Option	Can it inherit?	Example	Alternative
type → interface	No, not directly	`type B extends A` (error)	`type B = A & { ... }` (works)
interface → type	Yes, if type is an object	`interface B extends A {}` (works)	-

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