Typescript is Type for object property

Question

Is it possible?

const isNotValidated = <T>(req: NextApiRequest, res, schema: any): req.body is T => {}

NextApiRequest cannot be changed:

declare type NextApiRequest = IncomingMessage & {
    /**
     * Object of `query` values from url
     */
    query: {
        [key: string]: string | string[];
    };
    /**
     * Object of `cookies` from header
     */
    cookies: {
        [key: string]: string;
    };
    body: any;
};

Aleksey L. · Accepted Answer · 2019-12-26 14:55:52Z

2

There're no type guards for object properties, but you can define guard for whole object:

interface NextApiRequestEx<T> extends NextApiRequest {
  body: T;
}

type Bar = { bar: string };

const isNotValidated = <T>(req: NextApiRequest): req is NextApiRequestEx<T> => true;

declare const req: NextApiRequest;

if (isNotValidated<Bar>(req)) {
  req.body.bar // req.body is of type Bar here
}

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answered Dec 26, 2019 at 14:55

Aleksey L.

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