NestJS UnhandledPromiseRejectionWarning: How to Solve It

When working with Node.js and using frameworks like NestJS, encountering errors is a part of the development process. An example of a common error is the UnhandledPromiseRejectionWarning. This error can be a bit daunting if you don’t understand the underlying issue or how to handle it properly. So, let’s dive into what causes this error, and outline some strategies to resolve it.

Understanding the Error

The UnhandledPromiseRejectionWarning in NestJS is triggered when a Promise is rejected but not handled with a .catch() clause. In JavaScript and TypeScript, Promises are used to handle asynchronous operations, and a rejection occurs when an operation fails or an error is thrown. If the rejection isn’t captured by a rejection handler, it will bubble up to the console as a warning. This is a helpful warning, as it’s telling you that you should handle the error explicitly to prevent possible issues in your application.

Proper Error Handling

To resolve this warning, it’s crucial to implement proper error handling in the promises within your NestJS application. Using try...catch blocks around asynchronous code or adding .catch() handlers to your promises allows you to manage and respond to errors properly, preventing them from going unhandled. This kind of defensive coding helps in creating reliable and robust applications that can gracefully deal with unexpected conditions.

Implementing Error Handling in Your Code

Create a service or module where the problematic promise originates and observe your asynchronous operations. Make sure every promise has a corresponding .catch() method or a try...catch block. For example, consider you have a function to fetch user data in a service:

async function fetchUserData(userId: string): Promise<UserData> {
  try {
    const data = await this.externalService.getUserData(userId);
    return data;
  } catch (error) {
    // Handle your error here
    console.error('Error fetching user data:', error);
    throw new Error(error);
  }
}

It’s vital to not only catch errors but also to throw them when needed so that the caller of the function can handle them as well. For example, if you are working with controllers that utilize your service, ensure to handle possible rejections:

@Controller('users')
export class UserController {
  constructor(private readonly userService: UserService) {}

  @Get(':userId')
  async getUser(@Param('userId') userId: string) {
    try {
      const user = await this.userService.fetchUserData(userId);
      return user;
    } catch (error) {
      throw new HttpException('Failed to get user data', HttpStatus.BAD_REQUEST);
    }
  }
}

If you are handling errors in this way and still seeing the UnhandledPromiseRejectionWarning, it’s possible you have an unhandled rejection elsewhere in your code base. Use runtime diagnostics or linters to identify those.

Using Catch-All Error Filtering

Another solution is implementing a global error handling mechanism. In NestJS, filters can be used to catch exceptions that are thrown from anywhere within your application. Here’s an example of a global exception filter:

import { ExceptionFilter, Catch, ArgumentsHost, HttpException } from '@nestjs/common';
import { Request, Response } from 'express';

@Catch()
export class GlobalExceptionFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const request = ctx.getRequest<Request>();
    const response = ctx.getResponse<Response>();
    const status = exception instanceof HttpException
                   ? exception.getStatus()
                   : HttpStatus.INTERNAL_SERVER_ERROR;

    response.status(status).json({
      statusCode: status,
      timestamp: new Date().toISOString(),
      path: request.url,
    });
  }
}

Then, to engage the global filter, declare it within your main application file:

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalFilters(new GlobalExceptionFilter());
  await app.listen(3000);
}
bootstrap();

In conclusion, resolving UnhandledPromiseRejectionWarning in your NestJS application boils down to attentive coding practices that ensure every async operation is properly managed in terms of error handling. Through accurate placement of try...catch blocks, .catch() methods, and the utilization of global exception filters, you can significantly reduce the likelihood of unhandled promise rejections and improve the stability and reliability of your application.