apollo-server/docs/source/features/errors.md

---
title: Error handling
description: Making errors actionable on the client and server
---

Apollo server provides a couple predefined errors, including
`AuthenticationError`, `ForbiddenError`, `BadUserInputError` and a generic
`ApolloError`. These errors are designed to enhance errors thrown before and during GraphQL execution. The provided errors focus on debugging a Apollo server as well as enabling the client to take specific action based on an error.

When an error occurs in Apollo server both inside and outside of resolvers, each error inside of the `errors` array will contain an object at `extensions` that contains the information added by Apollo server.

## Default information

The first step to improving the usability of a server is providing the error stack trace by default. The following example demonstrates the response returned from Apollo server with a resolver that throws a node [`SystemError`](https://nodejs.org/api/errors.html#errors_system_errors).

```js line=14-16
const { 
  ApolloServer, 
  gql, 
} = require('apollo-server');

const typeDefs = gql`
  type Query {
    readError: String
  }
`;

const resolvers = {
  Query: {
    readError: (parent, args, context) => {
      fs.readFileSync('/does/not/exist');
    },
  },
};
```

The response will return:

![Screenshot demonstrating an error stacktrace and additional](../images/features/error-stacktrace.png)

> To disable stacktraces for production, pass `debug: false` to the Apollo server constructor or set the `NODE_ENV` environment variable to 'production' or 'test'

## Codes

In addition to stacktraces, Apollo Server's exported errors specify a human-readable string in the `code` field of `extensions` that enables the client to perform corrective actions. In addition to improving the client experience, the `code` field allows the server to categorize errors. For example, an `AuthenticationError` sets the code to `UNAUTHENTICATED`, which enables the client to reauthenticate and would generally be ignored as a server anomaly.

```js line=4,15-17
const { 
  ApolloServer, 
  gql, 
  AuthenticationError, 
} = require('apollo-server');

const typeDefs = gql`
  type Query {
    authenticationError: String
  }
`;

const resolvers = {
  Query: {
    authenticationError: (parent, args, context) => {
      throw new AuthenticationError('must authenticate');
    },
  },
};
```

The response will return:

![Screenshot demonstrating unauthenticated error code](../images/features/error-code.png)

## Augmenting error details

When clients provide bad input, you may want to return additional information 
like a localized message for each field or argument that was invalid. The 
following example demonstrates how you can use `BadUserInputError` to augment
your error messages with additional details.

```js line=15-21
const { 
  ApolloServer,
  BadUserInputError,
  gql,
} = require('apollo-server');

const typeDefs = gql`
  type Mutation {
    userInputError(input: String): String
  }
`;

const resolvers = {
  Mutation: {
    userInputError: (parent, args, context, info) => {
      if (args.input !== 'expected') {
        throw new BadUserInputError('Form Arguments invalid', {
          invalidArgs: Object.keys(args),
        });
      }
    },
  },
};
```

The response will return:

![Screenshot demonstrating augmented error](../images/features/error-user-input.png)

## Other errors

If you need to define other error codes that are specific to your
application, you can use the base `ApolloError` class.

```js
new ApolloError(message, code, additionalProperties);
``` 

## Masking and logging errors

The Apollo server constructor accepts a `formatError` function that is run on each error passed back to the client. This can be used to mask errors as well as logging.
This example demonstrates masking

```js line=4-7
const server = new ApolloServer({
  typeDefs,
  resolvers,
  formatError: error => {
    console.log(error);
    return new Error('Internal server error');
  },
});

server.listen().then(({ url }) => {
  console.log(`🚀 Server ready at ${url}`);
});
```