What steps should I take to fix the Typescript error showing up in my Next.js route?

import type { NextApiRequest, NextApiResponse } from "next";

import db from "../../app/libs/dbConn";

interface DataProps {
  auth: [
    {
      name?: string;
      email?: string;
      passwordHash?: string;
    }
  ];

  status: number;
  message: string;
}

export default async function handler(
  req: NextApiRequest,
  res: NextApiResponse<DataProps>
) {
  if (req.method === "POST") {
    const data = {
      name: "Gary",
      email: `gary@${Math.floor(Math.random() * 9999999)}.subdomain.com`,
      passwordHash: "-",
    };

    const user = await db.user.create({ data });

   return res.status(200).json({ auth: [data], status: 201, message: "created user" });
  }

  if (req.method === "GET") {
    const allUsers = await db.user.findMany();

    const users = allUsers.map((user) => {
      return {
        name: user.name || "",
        email: user.email || "",
        passwordHash: "" || "",
      };
    });

    return res.status(200).json({ auth: users, status: 200, message: "success" });
  }

  return res.status(404).json({
    auth: [{}],
    status: 405,
    message: "http verb not supported",
  });
}

In the "GET" part above, there is an error reported by Typescript for "auth". The error message states -- Type '{ name: string; email: string; passwordHash: string; }[]' is not assignable to type '[{ name?: string | undefined; email?: string | undefined; passwordHash?: string | undefined; }]'. Target requires 1 element(s) but source may have fewer.ts(2322) user.ts(6, 3): The expected type comes from property 'auth' which is declared here on type 'DataProps'

I cannot figure out what mistake I've made. Everything seems fine and works as intended when ignoring the Typescript error.

There's something missing that I can't seem to resolve.

Answer №1

The declaration of auth as

[ { name?: string; email?: string; passwordHash?: string; }]
signifies a single-element tuple containing exactly one
{ name?: string; email?: string; passwordHash?: string; }
element. However, the code expects
{ name?: string; email?: string; passwordHash?: string; }[]
, denoting an array with any number of
{ name?: string; email?: string; passwordHash?: string; }
elements. Therefore, modify DataProps.auth to
{ name?: string; email?: string; passwordHash?: string; }[]
for the code to function correctly.

For further insights, refer to the TypeScript Handbook's insights on arrays and tuples.

In a broader perspective, there are various approaches available for declaring types:

  • A 1-tuple (
    [{ name?: string; email?: string; passwordHash?: string; }]
    ) implies it always contains a single element. Though unconventional (if only one item is present, returning it directly as
    { name?: string; email?: string; passwordHash?: string }
    might be more suitable), there might be specific design requirements warranting such an approach.
  • An array (
    { name?: string; email?: string; passwordHash?: string }[]
    ) can accommodate 0, 1, or numerous items.
  • If all properties of a real User object need enforcing but an empty object should indicate an error, a union type would be utilized (
    ({ name: string; email: string; passwordHash: string } | Record<string, never>)[]
    ). (For details on Record<string, never>, consult here.)
  • Personally, instead of returning an empty object, considering making the entire auth property optional would be advisable:
    interface User {
      name: string;
      email: string;
      passwordHash: string;
    }
    
    interface DataProps {
      auth?: User[];
    
      status: number;
      message: string;
    }
    
    Or alternatively, employing a union for comprehensive typing for successful and erroneous responses would enhance clarity. This approach prevents accessing properties without verifying the success of the response, leveraging TypeScript's type system for enhanced code maintenance.
    interface DataProps {
      auth: User[];
    }
    interface ErrorResponse {
      status: number;
      message: string;
      error: true;
      errorDetails?: ErrorDetails; // for instance
    }
    type SuccessResponse<T> = T & {
      status: number;
      message: string;
      error?: false;
    }
    export default async function handler(
      req: NextApiRequest,
      res: NextApiResponse<DataProps>
    ) {
    

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