How to solve TS2322 error when doing this[prop] = data[prop]?

Question

The code below doesn't work and I got stuck. How to I fix this?

type Data = {
  id: number,
  name: string
}

class Person {
  id: number
  name: string

  constructor(personData: Data) {
    this.id = personData.id
    this.name = personData.name
  }

  update(updateData: Data) {
    for (const prop in updateData) {
      this[prop as keyof Data] = updateData[prop as keyof Data]
    }
  }
}

Playground link

the Error:

TS2322: Type 'string | number' is not assignable to type 'never'.   Type 'string' is not assignable to type 'never'.

The important line is in the update method: this[prop as keyof Data] = updateData[prop as keyof Data]. I suppose that typescript doesn't know that this[prop] and updateData[prop] are the same type (string|number). If all properties are the same type, there is no error. But how to tell typescript, that they are the of the same type?

Answer 1

Seems like it would be better to use composition here. That easily sidesteps the problem you're having.

type Data = {
  id: number,
  name: string
}

class Person {
  data: Data

  constructor(personData: Data) {
    this.data = personData
  }

  update(updateData: Data) {
    this.data = updateData
  }
}

If you really want to be able to access id and name as properties on a Person instance, you could add get accessors for them:

get id() { return this.data.id }

get name() { return this.data.name }

Playground