What is the reason for restricting a placeholder for an optional property in the interface to only be of type any?

Question

What is the reason for restricting a placeholder for an optional property in the interface to only be of type any?

I am facing a challenge with a file containing a single declaration, which is for an interface:

interface NamedPerson {
    firstName: string;
    age?: number;
    [propName: string]: any;
    greet(lastName: string): void;
}

Everything works perfectly as long as the type of [propName: string]: any. However, if I try to change it to number or any other type, I encounter this error:

[ts] Property 'greet' of type '(lastName: string) => void' is not assignable to string index type 'number'.

I specifically want any new unspecified property in an object that uses the NamedPerson interface to only allow values of type number, and nothing else.

Question: What needs to be corrected in the interface definition to meet my requirements?

typescript

Answer 1

Answer №1

It doesn't necessarily have to be of type any, but it must encompass all other field types. In this scenario, the type would need to include string (for firstName), number (for age), and (p: string) => void (for greet).

interface NamedPerson {
    firstName: string;
    age?: number;
    [propName: string]: string | number | ((p: string)=> void);
    greet(lastName: string): void;
}

In Typescript, consistency is enforced between the index return type and the known fields (since the index can return any of the known fields). You can introduce an inconsistent index by using an intersection type:

type NamedPerson = {
    firstName: string;
    age?: number;
    greet(lastName: string): void;
} & {
    [propName: string]: number;
};

The challenge lies in directly creating such an object; you can only achieve this using Object.assign along with a bit of type assertion:

let n: NamedPerson = Object.assign({
    firstName: "",
    age: 0,
    greet(lastName: string): void { }
}, {} as { [propName: string]: number });

Answer 2

It doesn't necessarily have to be of type any, but it must encompass all other field types. In this scenario, the type would need to include string (for firstName), number (for age), and (p: string) => void (for greet).

interface NamedPerson {
    firstName: string;
    age?: number;
    [propName: string]: string | number | ((p: string)=> void);
    greet(lastName: string): void;
}

In Typescript, consistency is enforced between the index return type and the known fields (since the index can return any of the known fields). You can introduce an inconsistent index by using an intersection type:

type NamedPerson = {
    firstName: string;
    age?: number;
    greet(lastName: string): void;
} & {
    [propName: string]: number;
};

The challenge lies in directly creating such an object; you can only achieve this using Object.assign along with a bit of type assertion:

let n: NamedPerson = Object.assign({
    firstName: "",
    age: 0,
    greet(lastName: string): void { }
}, {} as { [propName: string]: number });

What is the reason for restricting a placeholder for an optional property in the interface to only be of type any?

Answer №1

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