TypeScript TS2322: Type 'typeof Foo' is not assignable to type 'IFoo'

Question

I'm trying to compose some classes using ES2015 module syntax with TypeScript. Each class implements an interface in a .d.ts file.

Here is a MWE of the problem.

In a .d.ts file I have:

interface IBar {
  foo: IFoo;
  // ...
}

interface IFoo {
  someFunction(): void;
  // ...
}

My export is:

// file: foo.ts
export default class Foo implements IFoo {
  someFunction(): void {} 
  // ... 
}
// no errors yet.

And my import is:

import Foo from "./foo";

export class Bar implements IBar {
   foo: IFoo = Foo;
}

The error here is:

error TS2322: Type 'typeof Foo' is not assignable to type 'IFoo'.
Property 'someFunction' is missing in type 'typeof Foo'.

Any ideas here?

Answer 1

When you say foo: IFoo = Foo; you are assigning the class Foo to IFoo. However the interface IFoo is implemented by instances of that class. You need to do :

foo: IFoo = new Foo;