Mastering Error Handling: Throwing an Error from Apollo setContext and Catching in onError

If you’re a seasoned developer, you know the importance of error handling in your code. It’s like having a safety net that saves you from unexpected crashes and frustrating debugging sessions. And when it comes to Apollo Client, setContext is an essential feature that allows you to set context for your resolvers. But what happens when things go wrong? In this article, we’ll dive into the world of error handling and explore how to throw an error from Apollo setContext and catch it in onError.

The Problem: Apollo setContext and Error Handling

Imagine you’re building a robust application that relies heavily on GraphQL queries. You’ve set up Apollo Client, and everything seems to be working smoothly. But, what if your setContext resolver fails? Perhaps the authentication token is invalid, or the user doesn’t have the necessary permissions. If you don’t handle these errors correctly, your application will come crashing down.

import { ApolloClient, InMemoryCache, ApolloLink } from '@apollo/client';
import { setContext } from '@apollo/client/link/context';

const client = new ApolloClient({
  link: ApolloLink.from([
    setContext((request, previousContext) => {
      // Authentication token is invalid
      if (!localStorage.getItem('token')) {
        // What now?
      }
      return {
        headers: {
          Authorization: `Bearer ${localStorage.getItem('token')}`,
        },
      };
    }),
  ]),
  cache: new InMemoryCache(),
});

In the example above, if the authentication token is invalid, we’re left with a puzzle: how do we handle this error? We can’t just leave our application hanging; we need a way to notify the user or take alternative actions.

The Solution: Throwing an Error from setContext

The answer lies in throwing an error from the setContext resolver. By doing so, we can let Apollo Client know that something has gone wrong, and it will take care of propagating the error to the onError callback.

import { ApolloClient, InMemoryCache, ApolloLink } from '@apollo/client';
import { setContext } from '@apollo/client/link/context';

const client = new ApolloClient({
  link: ApolloLink.from([
    setContext((request, previousContext) => {
      // Authentication token is invalid
      if (!localStorage.getItem('token')) {
        throw new Error('Authentication token is invalid');
      }
      return {
        headers: {
          Authorization: `Bearer ${localStorage.getItem('token')}`,
        },
      };
    }),
  ]),
  cache: new InMemoryCache(),
});

By throwing an error, we’re signaling to Apollo Client that something has gone wrong, and it should propagate the error to the onError callback.

Catching the Error in onError

Now that we’ve thrown an error from setContext, we need to catch it in the onError callback. This is where the magic happens! The onError callback is called whenever an error occurs during the execution of a query or mutation.

import { ApolloClient, InMemoryCache, ApolloLink } from '@apollo/client';
import { setContext } from '@apollo/client/link/context';

const client = new ApolloClient({
  link: ApolloLink.from([
    setContext((request, previousContext) => {
      // Authentication token is invalid
      if (!localStorage.getItem('token')) {
        throw new Error('Authentication token is invalid');
      }
      return {
        headers: {
          Authorization: `Bearer ${localStorage.getItem('token')}`,
        },
      };
    }),
  ]),
  cache: new InMemoryCache(),
  onError: ({ networkError, graphQLErrors }) => {
    if (networkError) {
      console.log(`[Network Error]: ${networkError}`);
      // Handle network errors
    }
    if (graphQLErrors) {
      graphQLErrors.forEach(({ message, locations, path }) => {
        console.log(`[GraphQL Error]: ${message} in ${path} at ${locations}`);
        // Handle GraphQL errors
      });
    }
  },
});

In the onError callback, we can access the error object, which contains valuable information about the error that occurred. We can then use this information to notify the user, log the error, or take alternative actions.

Error Handling Best Practices

When it comes to error handling, there are some best practices to keep in mind:

• Be specific: Instead of throwing a generic error, be specific about what went wrong. This will help you and others understand the root cause of the issue.
• Provide context: Include as much context as possible when throwing an error. This can include information about the query, variables, or other relevant details.
• Catch and handle: Catch errors and handle them accordingly. Don’t let errors go unhandled, as they can lead to unexpected behavior or crashes.
• Log errors: Log errors in a centralized location, such as a logging service or analytics platform. This will help you track and debug issues.
• Notify the user: When an error occurs, notify the user in a friendly and informative manner. This can be done through toast notifications, error messages, or other means.

By following these best practices, you’ll be well on your way to creating a robust and error-free application.

Conclusion

In conclusion, throwing an error from Apollo setContext and catching it in onError is a crucial aspect of error handling in your GraphQL application. By following the instructions outlined in this article, you’ll be able to create a more robust and reliable application that can handle unexpected errors with ease.

Remember to be specific, provide context, catch and handle errors, log errors, and notify the user. By doing so, you’ll ensure that your application is always running smoothly, even in the face of adversity.

Happy coding, and may your errors be few and far between!

Keyword	Description
Throwing an error from Apollo setContext	Throwing an error from the setContext resolver to notify Apollo Client of an error.
Catching in onError	Catching the error in the onError callback to handle and notify the user of the error.

References:

• Apollo Client Documentation: setContext
• Apollo Client Documentation: onError

Related Articles

For more information on error handling and Apollo Client, check out these related articles:

• Error Handling in Apollo Client: A Comprehensive Guide
• Apollo Client Best Practices: Error Handling and Debugging
• GraphQL Error Handling: A Step-by-Step Guide

Thanks for reading, and I’ll catch you in the next article!

Frequently Asked Question

Get ready to throw and catch those errors like a pro! Here are the most frequently asked questions about throwing an error from Apollo setContext and catching in onError.