TypeScript: Cannot assign empty array to array of my own type

Question

I have an interface called Converter

export interface Converter<T> {
  uuid: CUUID;
  decode: (value: Buffer) => Promise<T>;
  encode?: (value: T) => Promise<Buffer>;
}

And I am using an array of that in another file:

import{ Converter } from "./characteristic";

export type Converters = Converter<any>[];

export default class Service<C extends Converters> {
  private converters: C;

  constructor(converters: C = []) {
           // ^^^^^^^^^^^^^^^^^^ error
    this.converters = converters;
  }
}

I am getting an error when I want to assign an empty array as an initial value to that property:

Type 'never[]' is not assignable to type 'C'.
  'never[]' is assignable to the constraint of type 'C', but 'C' could be instantiated with a different subtype of constraint 'Converter<any>[]'.ts(2322)

I have tried changing my type to

export type Converters = Converter<any>[] | [];

without any luck

I can do this:

export default class Service<C extends Converters> {
  private converters: C | [] = [];
}

but I am not sure why I have to or if that even is an appropriate solution and not just a anti-pattern workaround. I don't even understand what the problem is here.

Right off the top of my head I cannot see why it doesn't work. But it might be helpful to know which version of TypeScript you are using? — fredrik
– fredrik, Commented Sep 5, 2019 at 15:32
It doesn't work because you can specify a more specific type such as [Converter<string>] (a one-tuple type) for C, and you can't assign the value [] to a one-tuple. — jcalz
– jcalz, Commented Sep 5, 2019 at 18:28

Wilhelmina Lohan · Accepted Answer · 2019-09-05 15:55:14Z

1

The following seems to work:

export interface Converter<T = any> {
  uuid: CUUID;
  decode: (value: Buffer) => Promise<T>;
  encode?: (value: T) => Promise<Buffer>;
}

export default class Service<C extends Converter> {
  private converters: C[];

  constructor(converters: C[] = []) {
    this.converters = converters;
  }
}

I think export type Converters = Converter<any>[]; must obscure that it is an array so it tries to compare never[] to Converters instead of [] (with possibility to .push(Converter)) to Converter[].

Edit: actually if the the Service is intended to only have Converters of a certain type not mixed (I don't know) it might be better to do:

export interface Converter<T> {
  uuid: CUUID;
  decode: (value: Buffer) => Promise<T>;
  encode?: (value: T) => Promise<Buffer>;
}

export default class Service<T> {
  private converters: Converter<T>[];

  constructor(converters: Converter<T>[] = []) {
    this.converters = converters;
  }
}

edited Sep 5, 2019 at 15:55

answered Sep 5, 2019 at 15:39

Wilhelmina Lohan

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3 Comments

Lukas Over a year ago

Hey William. Thank you, but this won't work for me. The Converters can have multiple types.

jcalz Over a year ago

"This won't work for me" ... Wait, why doesn't the first solution given here with types like class Service<C extends Converter> { converters: C[]; constructor(converters: C[] = []) { ... }} work for you? C can be a union. If it really doesn't work I think we need a minimal reproducible example that shows why.

Lukas Over a year ago

I'm sorry, I was assuming that the first the solution will work on an array of the same converter type, which I see now it is not. I kept working on my "thing" and have created another, more elaborate question where I am tacking the problem. I will close this question in favor of this one: stackoverflow.com/questions/57812051/… Sorry for the effort you have already put into this, but the other one is probably more precisely what I am trying to do

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