Typescript, class type is not assignable to index interface

Question

I have a class that has some public properties and I would like to pass it to a function with a generic argument of an index interface type, but I get an compiler error that says Argument of type 'Car' is not assignable to parameter of type '{ [k: string]: unknown; }'.

Could someone explain what is wrong because I assume all Classes are of type object?!!

class Car {

  name: string | undefined;

}

function toStr <T extends { [k: string]: unknown }>(i: T) {
  return i.toString();
}


const jeep = new Car();
jeep.name = 'jeep';

toStr(jeep);

Car doesn't define index signature but function parameter has constraint for it. Could you explain what's the point of generics here? Both function toStr(i: object) ... and function toStr(i: Record<string, any>) ... will work — Aleksey L.
– Aleksey L., Commented Mar 13, 2021 at 13:05
Or accept any type with a toString method (should be everything) toStr(i: {toString(): string}): string — Linda Paiste
– Linda Paiste, Commented Mar 13, 2021 at 14:03
@LindaPaiste technically not everything will have toString (e.g. Object.create(null)), but typescript can't catch this — Aleksey L.
– Aleksey L., Commented Mar 14, 2021 at 5:36
The point is eslint complains when you use the object type. — alireza-bonab
– alireza-bonab, Commented Apr 7, 2021 at 6:50

captain-yossarian from Ukraine · Accepted Answer · 2021-03-13 14:47:25Z

1

Just use object instead { [k: string]: unknown }.

class Car {

  name: string | undefined;

}

function toStr <T extends object>(i: T) {
  return i.toString();
}


const jeep = new Car();
jeep.name = 'jeep';

const result = toStr(jeep);

answered Mar 13, 2021 at 14:47

captain-yossarian from Ukraine

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