Handle Prisma errors with Express

Question

I am having some issues with error handling using ExpressJS and Prisma. Whenever a Prisma Exception occurs, my entire Node application crashes, and I have to restart it. I have done some googling and have looked at the Prisma Docs for error handling, but I can't find any answers.

I know I could possibly use try and catch, but this feels unnecessary, as I could handle this much better with an error handler, especially when a lot of information on errors is passed through Prisma.

I have tried to implement the Express error handler like this:

// index.ts

import errorHandler from "./middleware/errorHandler";
...
server.use(errorHandler);

// errorHandler.ts

import { NextFunction, Response } from "express";

// ts-ignore because next function is required for some weird reason
// @ts-ignore
const errorHandler = (err: any, _: any, res: Response, next: NextFunction) => {
    console.error(err.stack);
    res.status(500).send("Internal Server Error");
};

export default errorHandler;

This works fine for normal errors, but doesn't execute for Prisma errors, but instead just crashes the Node application.

How can I implement an error handler so I can manage Prisma Expections?

user_8270508 · Answer

I ran into this problem today and I couldn't find answers too. I believe we'll have to write our own custom error handler for Prisma exceptions and not throw the error.

try {
  await prismaOperation();
} catch(e) {
  throw e; // avoid this which will crash our app
  /* Process Prisma error with error codes
     and prepare an appropriate error message
  */
  const error = prismaCustomErrorHandler(e);
  res.send(error); // Sending response instead of passing it to our default handler
}

Also,

// ts-ignore because next function is required for some weird reason

In Express, error-handling functions have 4 arguments instead of 3: (err, req, res, next).

Express interprets a middleware function with 3 arguments as (req, res, next) which is different from, had you ommited the 4th argument, (err, _, res). Hence, Express won't pass any error and your err will be a req object, _ (req) a res object and res a next function.

Edit:

...
const error = prismaCustomErrorHandler(e);
  res.send(error); // Sending response instead of passing it to our default handler
...

// Edit: Or you could process and pass the error using `next(error)` to default error handler.

The above method won't crash the app and indeed sends the response. But whether you use next or res.send, errors have to be processed.

Handle Prisma errors with Express

1 Answers