NestJS Unhandle Promise Rejection Warning – Solutions

Understanding the Unhandled Promise Rejection in NestJS

An Unhandled Promise Rejection occurs in a Node.js environment, such as NestJS, when a promise is rejected but not handled by a .catch() method or within a try...catch block in an asynchronous function. In the context of NestJS, which is a Node.js framework for building efficient and scalable server-side applications, this warning message can disrupt the flow of an application and might lead to further errors that may be harder to trace if not correctly handled.

Common reasons for this warning to occur in NestJS include trying to access a database or an external API where the operation fails, for example, due to network issues or bad data, but the error is not properly caught within the promise chain. The absence of proper error handling causes the Node.js runtime to print this verbal warning to the console to signal that something might be off in your code handling logic.

Correct Error Handling in Promises

To address the issue, the initial step is to ensure that all promises have proper error handling mechanisms. This might involve patching up all the promises in your NestJS service or module with .catch() blocks or wrapping asynchronous calls in try...catch, depending on the context.

Following the asynchronous/await syntax which is part of modern JavaScript, you might want to make asynchronous functions and wait for the execution of promises within a try...catch block. This is a neat way to handle both the resolved and rejected state of a promise in a synchronous-looking way.

Implementing Proper Error Handling

To demonstrate proper error handling in NestJS with an example:

import { Injectable } from '@nestjs/common';
import { HttpClientService } from './http-client.service';

@Injectable()
export class SomeService {
  constructor(private httpClientService: HttpClientService) {}

  async fetchData(): Promise {
    try {
      const responseData = await this.httpClientService.get('https://some-api.com/data');
      return responseData.data;
    } catch (error) {
      // Here you can handle the exception in a meaningful way
      console.error('Error fetching data:', error);
      throw new CustomException('Error fetching data', error);
    }
  }
}

In the above example, our SomeService is making use of an HttpClientService to fetch data from an external API. The fetching is wrapped in a try...catch block, meaning if the promised data is rejected or any error occurs during the await operation, the catch block will handle the error and not result in an Unhandled Promise Rejection.

Handling Rejections in Promise Chains

For scenarios where you might not be using async/await, you can add a .catch() method at the end of your promise chain:

this.someService.processData()
  .then((result) => {
    // Handle successful result
  })
  .catch((error) => {
    // Error handling logic here
    console.error('There was an error processing data', error);
  });

This ensures that if the processData() method returns a promise that gets rejected, the catch block attached to the end of the promise chain will handle it and won’t lead to an Unhandled Promise Rejection issue.

Using Global Exception Filters in NestJS

Beyond manual handling of rejections at each promise, in NestJS, you can also use Global Exception Filters to catch and handle exceptions throughout the application consistently. These filters can catch any unhandled exceptions thrown during the processing of a request, thus being a safety net for errors missed by the local error handling logic.

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

@Catch()
export class GlobalExceptionsFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    // Implementation to handle the exception
    // Respond with custom error message, log error, etc.
  }
}

To activate the Global Exceptions Filter, it must be declared in the main file of your NestJS application, often the main.ts:

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { GlobalExceptionsFilter } from './global-exceptions.filter';

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

Final Thoughts

Handling promises properly and ensuring that all rejected states are accounted for goes a long way in maintaining the stability and reliability of your NestJS application. By implementing try/catch in async functions, using .catch() with promise chains, and setting up global exception filters, you can avoid the pesky ‘Unhandled Promise Rejection’ warnings and ensure a smoother application flow.