How to: Next.js API Global Errors & Auth Middleware

#nextjs #typescript #errors #authentication

This one's going to be quick. no bs. I was working on a Next.js project and needed a way to handle API Route Errors Globally. Similar to Express in Node.js.
This is what I've come up with and it works wonders for my setup.

We create a handler function that will take multiple handlers and run them one by one.

import { ApiError } from "next/dist/server/api-utils";
import { NextResponse, NextRequest } from "next/server";

export const custom_middleware =
  (...handlers: Function[]) =>
  async (req: NextRequest, res: NextResponse) => {
    try {
      for (const handler of handlers) {
        await handler(req, res);
      }
    } catch (error) {
      if (error instanceof ApiError) {
        return NextResponse.json(
          { message: error.message },
          { status: error.statusCode }
        );
      } else {
        /// Log server errors using winston or your preferred logger
        console.log(error);
        return NextResponse.json(
          { message: "Server died for some reason" },
          { status: 500 }
        );
      }
    }
  };

Now we can add it to a route

/// app/api/ping/route.ts

import { custom_middleware } from "@/app/lib/server/middleware";
import { ApiError } from "next/dist/server/api-utils";
import { NextRequest, NextResponse } from "next/server";

const main_handler = (req: NextRequest, res: NextResponse) => {
  const isAuthenticated = false;

  if (isAuthenticated) {
    return NextResponse.json({ success: true });
  }

  throw new ApiError(400, "Some error");
};

export const GET = custom_middleware(main_handler);

We can see the fruits of our labour in the browser itself.
We can add our own custom authentication logic here too.

export const auth_middleware = async (req: NextRequest, res: NextResponse) => {
  /// Your auth logic
  const isAuthenticated = false;
  if (!isAuthenticated) {
    throw new ApiError(401, "Unauthorized");
  }
};

Call it in our middleware_handler

export const custom_middleware =
  (...handlers: Function[]) =>
  async (req: NextRequest, res: NextResponse) => {
    try {
      ///
      /// Auth middleware
      await auth_middleware(req, res);

      for (const handler of handlers) {
        await handler(req, res);
      }
    } catch (error) {
      if (error instanceof ApiError) {
        return NextResponse.json(
          { message: error.message },
          { status: error.statusCode }
        );
      } else {
        /// Log server errors using winston or your preferred logger
        console.log(error);
        return NextResponse.json(
          { message: "Server died for some reason" },
          { status: 500 }
        );
      }
    }
  };