Typescript this infered type unable to get the type of parent super class

user_5069957 0
Viewed 50

I have a child class that inherit from a parent class. The child class this is unable to get the typing of the parent class

class Foo {
  foo: string = ''
  merge(opts: Partial<this>): this { return this }
}

class Bar extends Foo {
  bar() {
    return this.merge({ // typing error because foo is not declared as a property of Bar
      foo: '3'
    })
  }
}
typescript
3 Answers

You have at least 2 options:

  1. The first would be to override the method in the inheriting classes
class Foo {
  foo: string = ''
  merge(opts: Partial<Foo>): Partial<Foo> { return this }
}

class Bar extends Foo {
  asd: string = 'qweqwe';

  merge(opts: Partial<Bar>): Partial<Bar> { return super.merge(opts); }

  bar() {
    return this.merge({
      foo: '3',
      asd: "ceva"
    })
  }
}

let a = new Foo()
console.log(a.merge({}))

let b = new Bar()
console.log(b.bar())
  1. Second would be to create a generic merge method that will need to be passed a type at the call site. Something similar to this.
class Foo {
  foo: string = ''
  merge<U extends Foo>(opts: Partial<U>): Partial<U> { return this as unknown as Partial<U> }
}

class Bar extends Foo {
  asd: string = 'qweqwe';

  bar() {
    return this.merge<Bar>({
      foo: '3',
      asd: "ceva"
    })
  }
}

let a = new Bar()
console.log(a.bar())
user_20546020

You should not use this as a type in this case. Replace it with the utility ThisType.

class Foo {
  foo: string = ''
  merge(opts: Partial<ThisType<Foo>>): ThisType<Foo> { return this }
}

Playground

user_86136300

One option is to pass the child class as a generic argument to the base class:

class Foo<T extends Foo<T>> {
  foo: string = ''
  merge(opts: Partial<T>): Partial<Foo<T>> { return this }
}

class Bar extends Foo<Bar> {
  x: string = ''
  bar() {
    return this.merge({
      foo: '3',
      x: 'a'
    })
  }
}
user_11001580
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