Index signature required in interface but not in object type

[calebmer]

TypeScript Version: 3.3.3

Search Terms: index signature, interface, object type

Code

interface A {
    p: number,
    q: string,
}

type B = {
    p: number,
    q: string,
};

type JSONObjectValue = {
    [key: string]: number | string,
};

declare const a: A;
declare const b: B;

const jsonA: JSONObjectValue = a;
const jsonB: JSONObjectValue = b;

Expected behavior: Both a should be assignable to jsonA and b should be assignable to jsonB.

Actual behavior: jsonA = a errs but jsonB = b does not err.

Playground Link

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Use case: I have a type JSONValue (which I’ve seen before in some DefinitelyTyped libraries) to make sure an object is a valid JSON value. Which would mean it doesn’t contain functions or other problematic properties.

export type JSONValue = null | boolean | number | string | JSONArrayValue | JSONObjectValue;
export interface JSONArrayValue extends ReadonlyArray<JSONValue> {}
export type JSONObjectValue = {readonly [key: string]: JSONValue};

I also have some interfaces which inherit from each other using extends which should be valid JSON values. Which is why I would prefer to use an interface over an object type in this case.

[jack-williams]

Is this covered by #15300?

[calebmer]

Yes it is!