Consider the TypeScript code snippet below:
type Primitive = undefined | null | boolean | number | string;
// A POJO is simply meant to represent a basic object, without any complexities, where the content is unknown.
interface POJO {
[key: string]: Primitive | POJO;
}
The POJO here serves as a generic object, intended for simple usage like converting it to JSON or similar processes. However, an issue arises in the following code block:
// Implementation of certain functions elsewhere...
declare function post(path: string, data: POJO): Promise<Response>;
declare function getToken(): string;
type LinkOrMessage = {link: string} | {message: string};
function specialPost(path: string, data: LinkOrMessage): Promise<Response> {
const body = {
...data,
token: getToken(),
};
// Error explanation provided below...
return post(path, body);
}
The error message states:
Argument of type '{ token: string; link: string; } | { token: string; message: string; }'
is not assignable to parameter of type 'POJO'.
Type '{ token: string; link: string; }'
is not assignable to type 'POJO'.
Index signature is missing in type '{ token: string; link: string; }'
This prompts confusion. Interestingly, simplifying the LinkOrMessage type by selecting only {link: string} or {message: string} resolves the error. Each individual type in the union seems compatible with POJO, yet the union itself poses an issue?
If anyone could provide insight into this behavior, it would be greatly appreciated.