Handling operation errors

Apollo Client can encounter a variety of errors when executing operations on your GraphQL server. Apollo Client helps you handle these errors according to their type, enabling you to show appropriate information to the user when an error occurs.

Error types

Executing GraphQL operations on a remote server can result in GraphQL errors or network errors.

GraphQL errors

These are errors related to the server-side execution of a GraphQL operation. They include:

• Syntax errors (e.g., a query was malformed)
• Validation errors (e.g., a query included a schema field that doesn't exist)
• Resolver errors (e.g., an error occurred while attempting to populate a query field)

If a syntax error or validation error occurs, your server doesn't execute the operation at all because it's invalid. If resolver errors occur, your server can still return partial data.

If a GraphQL error occurs, your server includes it in the errors array of its response to Apollo Client:

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{
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  "errors": [
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    {
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      "message": "Cannot query field \"nonexistentField\" on type \"Query\".",
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      "locations": [
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        {
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          "line": 2,
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          "column": 3
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        }
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      ],
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      "extensions": {
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        "code": "GRAPHQL_VALIDATION_FAILED",
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        "exception": {
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          "stacktrace": [
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            "GraphQLError: Cannot query field \"nonexistentField\" on type \"Query\".",
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            "...additional lines..."
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          ]
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        }
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      }
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    }
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  ],
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  "data": null
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}

Apollo Client then adds those errors to the error.graphQLErrors array returned by your useQuery call (or whichever operation hook you used).

If a GraphQL error prevents Apollo Server from executing your operation at all, it responds with a 4xx status code. Apollo Server responds with a 200 status code if resolver errors occurred but the response still includes partial data.

Partial data with resolver errors

An operation that produces resolver errors might also return partial data. This means that some (but not all) of the data your operation requested is included in your server's response. Apollo Client ignores partial data by default, but you can override this behavior by setting a GraphQL error policy.

Network errors

These are errors encountered while attempting to communicate with your GraphQL server, usually resulting in a 4xx or 5xx response status code (and no data).

When a network error occurs, Apollo Client adds it to the error.networkError field returned by your useQuery call (or whichever operation hook you used).

You can add retry logic and other advanced network error handling to your application with Apollo Link.

GraphQL error policies

If a GraphQL operation produces one or more resolver errors, your server's response might still include partial data in the data field:

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{
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  "data": {
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    "getInt": 12,
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    "getString": null
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  },
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  "errors": [
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    {
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      "message": "Failed to get string!"
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      // ...additional fields...
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    }
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  ]
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}

By default, Apollo Client throws away partial data and populates the error.graphQLErrors array of your useQuery call (or whichever hook you're using). You can instead use these partial results by defining an error policy for your operation.

Apollo Client supports the following error policies for an operation:

Setting an error policy

Specify an error policy in the options object you provide your operation hook (such as useQuery), like so:

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const MY_QUERY = gql`
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  query WillFail {
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    badField # This field's resolver produces an error
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    goodField # This field is populated successfully
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  }
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`;
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function ShowingSomeErrors() {
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  const { loading, error, data } = useQuery(MY_QUERY, { errorPolicy: "all" });
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  if (loading) return <span>loading...</span>;
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  return (
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    <div>
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      <h2>Good: {data.goodField}</h2>
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      <pre>
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        Bad:{" "}
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        {error.graphQLErrors.map(({ message }, i) => (
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          <span key={i}>{message}</span>
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        ))}
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      </pre>
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    </div>
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  );
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}

This example uses the all error policy to render both partial data and error information whenever applicable.

Advanced error handling with Apollo Link

The Apollo Link library enables you to configure advanced handling of errors that occur while executing GraphQL operations.

As a recommended first step, you can add an onError link to your link chain that receives error details and acts on them accordingly.

The example below passes the ApolloClient constructor a link chain with two links:

• An onError link that checks for graphQLErrors or a networkError in the server's response. It logs the details of whichever error(s) it finds.
• An HttpLink that sends each GraphQL operation to your server.
  • This is the chain's terminating link.

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import { ApolloClient, HttpLink, InMemoryCache, from } from "@apollo/client";
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import { onError } from "@apollo/client/link/error";
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const errorLink = onError(({ graphQLErrors, networkError }) => {
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  if (graphQLErrors)
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    graphQLErrors.forEach(({ message, locations, path }) =>
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      console.log(
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        `[GraphQL error]: Message: ${message}, Location: ${locations}, Path: ${path}`
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      )
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    );
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  if (networkError) console.error(`[Network error]: ${networkError}`);
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});
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const httpLink = new HttpLink({ uri: 'http://localhost:4000/graphql' })
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const client = new ApolloClient({
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  cache: new InMemoryCache(),
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  link: from([errorLink, httpLink]),
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});

Retrying operations

Apollo Link helps you retry failed operations that might be resolved by a followup attempt. We recommend different links depending on the type of error that occurred:

• The onError link for GraphQL errors
• The RetryLink for network errors

On GraphQL errors

The onError link can retry a failed operation based on the type of GraphQL error that's returned. For example, when using token-based authentication, you might want to automatically handle re-authentication when the token expires.

To retry an operation, you return forward(operation) in your onError function. Here's an example:

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onError(({ graphQLErrors, networkError, operation, forward }) => {
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  if (graphQLErrors) {
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    for (let err of graphQLErrors) {
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      switch (err.extensions.code) {
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        // Apollo Server sets code to UNAUTHENTICATED
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        // when an AuthenticationError is thrown in a resolver
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        case "UNAUTHENTICATED":
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          // Modify the operation context with a new token
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          const oldHeaders = operation.getContext().headers;
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          operation.setContext({
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            headers: {
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              ...oldHeaders,
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              authorization: getNewToken(),
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            },
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          });
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          // Retry the request, returning the new observable
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          return forward(operation);
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      }
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    }
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  }
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  // To retry on network errors, we recommend the RetryLink
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  // instead of the onError link. This just logs the error.
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  if (networkError) {
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    console.log(`[Network error]: ${networkError}`);
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  }
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});

If you don't want to retry an operation, your onError link's function should return nothing.

On network errors

To retry operations that encounter a network error, we recommend adding a RetryLink to your link chain. This link enables you to configure retry logic like exponential backoff and total number of attempts.

See the documentation for RetryLink.

onError link options

See the onError API reference.