typescript

How to make class extend interface?

Given

interface Foo {
  x: number
  fn?(): void
}

class Bar implements Foo {
  x = 1
}

const bar = new Bar()
bar.fn

The last line errors: bar does not appear to be a Foo.

Solution

  • I solved it as I was asking it:

    You have to add interface Bar extends Foo {} like so:

    interface Foo {
  x: number
  fn?(): void
}

class Bar implements Foo {
  x = 1
}

interface Bar extends Foo {}

const bar = new Bar()
bar.fn // <-- works now