Queries

This article shows how to fetch GraphQL data in React with the useQuery hook and attach the result to your UI. You'll also learn how Apollo Client simplifies data management code by tracking error and loading states for you.

Prerequisites

This article assumes you're familiar with building basic GraphQL queries. If you need a refresher, we recommend this guide. You can also build example queries against Apollo's full-stack tutorial server.

This article also assumes that you've already set up Apollo Client and have wrapped your React app in an ApolloProvider component. For more information, see the getting started guide.

To follow along with the examples below, open up our starter project and sample GraphQL server on CodeSandbox. You can view the completed version of the app here.

Executing a query

The useQuery React hook is the primary API for executing queries in a non-suspenseful Apollo application (for information about using Apollo Client with suspenseful React, see Suspense). To run a query within a React component, call useQuery and pass it a GraphQL document. When your component renders, useQuery returns an object from Apollo Client that contains loading, error, dataState and data properties you can use to render your UI.

Let's look at an example. First, we'll create a GraphQL query named GET_DOGS. Remember to wrap query strings in the gql function to parse them into query documents:

1import { gql } from "@apollo/client";
2import { useQuery } from "@apollo/client/react";
3
4const GET_DOGS = gql`
5  query GetDogs {
6    dogs {
7      id
8      breed
9    }
10  }
11`;

Next, we'll create a component named Dogs. Inside it, we'll pass our GET_DOGS query to the useQuery hook:

1function Dogs({ onDogSelected }) {
2  const { loading, error, data } = useQuery(GET_DOGS);
3
4  if (loading) return "Loading...";
5  if (error) return `Error! ${error.message}`;
6
7  return (
8    <select name="dog" onChange={onDogSelected}>
9      {data.dogs.map((dog) => (
10        <option key={dog.id} value={dog.breed}>
11          {dog.breed}
12        </option>
13      ))}
14    </select>
15  );
16}

As our query executes and the values of loading, error, and data change, the Dogs component can intelligently render different UI elements according to the query's state:

• As long as loading is true (indicating the query is still in flight), the component presents a Loading... notice.

• When loading is false and there is no error, the query has completed. The component renders a dropdown menu that's populated with the list of dog breeds returned by the server.

When the user selects a dog breed from the populated dropdown, the selection is sent to the parent component via the provided onDogSelected function.

In the next step, we'll associate the dropdown with a more sophisticated query that uses GraphQL variables.

Caching query results

Whenever Apollo Client fetches query results from your server, it automatically caches those results locally. This makes later executions of that same query extremely fast.

To see this caching in action, let's build a new component called DogPhoto. DogPhoto accepts a prop called breed that reflects the current value of the dropdown menu in our Dogs component:

1const GET_DOG_PHOTO = gql`
2  query Dog($breed: String!) {
3    dog(breed: $breed) {
4      id
5      displayImage
6    }
7  }
8`;
9
10function DogPhoto({ breed }) {
11  const { loading, error, data } = useQuery(GET_DOG_PHOTO, {
12    variables: { breed },
13  });
14
15  if (loading) return null;
16  if (error) return `Error! ${error}`;
17
18  return (
19    <img src={data.dog.displayImage} style={{ height: 100, width: 100 }} />
20  );
21}

Notice that we're providing a configuration option (variables) to the useQuery hook this time. The variables option is an object that contains all of the variables we want to pass to our GraphQL query. In this case, we want to pass the currently selected breed from the dropdown.

Select bulldog from the dropdown to see its photo appear. Then switch to another breed, and then switch back to bulldog. You'll notice that the bulldog photo loads instantly the second time around. This is the cache at work!

Next, let's learn some techniques for ensuring that our cached data is fresh.

Updating cached query results

Sometimes, you want to make sure that your query's cached data is up to date with your server's data. Apollo Client supports two strategies for this: polling and refetching.

Polling

Polling provides near-real-time synchronization with your server by executing your query periodically at a specified interval. To enable polling for a query, pass a pollInterval configuration option to the useQuery hook with an interval in milliseconds:

1function DogPhoto({ breed }) {
2  const { loading, error, data } = useQuery(GET_DOG_PHOTO, {
3    variables: { breed },
4    pollInterval: 500,
5  });
6
7  if (loading) return null;
8  if (error) return `Error! ${error}`;
9
10  return (
11    <img src={data.dog.displayImage} style={{ height: 100, width: 100 }} />
12  );
13}

By setting pollInterval to 500, we fetch the current breed's image from the server every 0.5 seconds. Note that if you set pollInterval to 0, the query does not poll.

You can also start and stop polling dynamically with the startPolling and stopPolling functions that are returned by the useQuery hook. When using these functions, set the pollInterval configuration option as a parameter of the startPolling function.

Refetching

Refetching enables you to refresh query results in response to a particular user action, as opposed to using a fixed interval.

Let's add a button to our DogPhoto component that calls our query's refetch function whenever it's clicked.

You can optionally provide a new variables object to the refetch function. If you don't provide a variables object, the query uses the same variables object that it used in its previous execution.

1function DogPhoto({ breed }) {
2  const { loading, error, data, refetch } = useQuery(GET_DOG_PHOTO, {
3    variables: { breed },
4  });
5
6  if (loading) return null;
7  if (error) return `Error! ${error}`;
8
9  return (
10    <div>
11      <img src={data.dog.displayImage} style={{ height: 100, width: 100 }} />
12      <button onClick={() => refetch()}>Refetch new breed!</button>
13    </div>
14  );
15}

Click the button and notice that the UI updates with a new dog photo. Refetching is an excellent way to guarantee fresh data, but it introduces some complexity with loading state. In the next section, we'll cover strategies for handling complex loading and error state.

Providing new variables to refetch

You call refetch with a new set of variables like so:

1<button
2  onClick={() =>
3    refetch({
4      breed: "dalmatian", // Always refetches a dalmatian instead of original breed
5    })
6  }
7>
8  Refetch!
9</button>;

If you provide new values for some of your original query's variables but not all of them, refetch uses each omitted variable's original value.

Inspecting loading states

We've already seen that the useQuery hook exposes our query's current loading state. This is helpful when a query first loads, but what happens to our loading state when we're refetching or polling?

Let's return to our refetching example from the previous section. If you click the refetch button, you'll see that the component re-renders while we're refetching the photo.

The useQuery hook's result object provides fine-grained information about the status of the query via the networkStatus property. This allows us to indicate to the user that we're refetching the photo:

1import { NetworkStatus } from "@apollo/client";
2
3function DogPhoto({ breed }) {
4  const { loading, error, data, refetch, networkStatus } = useQuery(
5    GET_DOG_PHOTO,
6    {
7      variables: { breed },
8    }
9  );
10
11  if (networkStatus === NetworkStatus.refetch) return "Refetching!";
12  if (loading) return null;
13  if (error) return `Error! ${error}`;
14
15  return (
16    <div>
17      <img src={data.dog.displayImage} style={{ height: 100, width: 100 }} />
18      <button onClick={() => refetch()}>Refetch!</button>
19    </div>
20  );
21}

The value of loading also updates accordingly. Even if you don't use the more fine-grained information provided by the networkStatus property, the loading property indicates that the refetch is in progress.

The networkStatus property is a NetworkStatus enum that represents different loading states. Refetch is represented by NetworkStatus.refetch. There are additional values for e.g. polling and pagination. For a full list of all the possible loading states, check out this table.

1const client = new ApolloClient({
2  // ... other options
3  defaultOptions: {
4    watchQuery: {
5      notifyOnNetworkStatusChange: false,
6    },
7  },
8});

To view a complete version of the app we just built, check out the CodeSandbox here.

Inspecting error states

You can customize your query error handling by providing the errorPolicy configuration option to the useQuery hook. The default value is none, which tells Apollo Client to treat all GraphQL errors as runtime errors. In this case, Apollo Client discards any query response data returned by the server and sets the error property in the useQuery result object.

If you set errorPolicy to all, useQuery does not discard query response data, allowing you to render partial results.

For more information, see Handling operation errors.

Fetching in response to user interaction

When React renders a component that calls useQuery, Apollo Client automatically executes the corresponding query. But what if you want to execute a query in response to a user interaction, such as a user clicking a button?

The useLazyQuery hook is for manually executing queries. Unlike useQuery, when you use useLazyQuery, it doesn't immediately execute its associated query. Instead, it returns an execution function that you call whenever you need to execute the query.

Here's an example:

1import React from "react";
2import { useLazyQuery } from "@apollo/client/react";
3
4function GetDogsOnClick() {
5  const [getDogs, { loading, error, data }] = useLazyQuery(GET_DOGS);
6
7  if (loading) return <p>Loading ...</p>;
8  if (error) return `Error! ${error.message}`;
9
10  return (
11    <div>
12      {data?.dogs.map((dog) => (
13        <Dog key={dog.id} dog={dog} />
14      ))}
15      <button onClick={() => getDogs()}>Load dogs</button>
16    </div>
17  );
18}

The first item in useLazyQuery's return tuple is the execution function, and the second item is an object that contains information about the executed query, such as the loading, error, data, and dataState properties.

Re-rendering with new options

Unlike useQuery, options provided to useLazyQuery that change on re-renders do not automatically execute the query. Instead, useLazyQuery waits to execute the query with the updated options until you call the execution function again.

The following is an example that changes the fetch policy depending on whether the user is online or offline:

1function GetDogs({ isOnline }) {
2  const [getDogs, { loading, data }] = useLazyQuery(GET_DOGS, {
3    fetchPolicy: isOnline ? "network-only" : "cache-only",
4  });
5
6  if (loading) return <p>Loading ...</p>;
7
8  return (
9    <div>
10      {data && <Dogs data={data.dogs} />}
11      <button onClick={() => getDogs()}>Get dogs</button>
12    </div>
13  );
14}

Working with variables

You provide variables to the execution function when executing the query.

The following example gets a specific dog's photo when you click the Get photo button:

1function DogPhoto() {
2  const [getDogPhoto, { loading, error, data }] = useLazyQuery(GET_DOG_PHOTO);
3
4  if (loading) return <p>Loading ...</p>;
5  if (error) return `Error! ${error.message}`;
6
7  return (
8    <div>
9      {data?.dog && <img src={data.dog.displayImage} />}
10      <button onClick={() => getDogPhoto({ variables: { name: "Lucky" } })}>
11        Get photo
12      </button>
13    </div>
14  );
15}

Changing variables

You change variables by calling the execution function with updated variables.

The following example gets the selected dog's photo when you click the Get photo button.

1function DogPhoto() {
2  const [selectedDog, setSelectedDog] = useState(null);
3  const [getDogPhoto, { loading, error, data }] = useLazyQuery(GET_DOG_PHOTO);
4
5  if (loading) return <p>Loading ...</p>;
6  if (error) return `Error! ${error.message}`;
7
8  return (
9    <div>
10      <DogSelect
11        onChange={(dogName) => setSelectedDog(dogName)}
12        value={selectedDog}
13      />
14      {data?.dog && <img src={data.dog.displayImage} />}
15      {selectedDog && (
16        <button
17          onClick={() => getDogPhoto({ variables: { name: selectedDog } })}
18        >
19          Get photo
20        </button>
21      )}
22    </div>
23  );
24}

If you need to reference the currently executed query's variables, use the variables property returned by useLazyQuery. The variables property contains the value of the variables from the last execution of the query.

1function DogPhoto() {
2  const [getDogPhoto, { called, variables }] = useLazyQuery(GET_DOG_PHOTO);
3
4  return (
5    <div>
6      {/* ... */}
7      <button onClick={() => getDogPhoto({ variables: { name: selectedDog } })}>
8        Get photo
9      </button>
10
11      {called && <span>Last fetched: {variables.name}</span>}
12    </div>
13  );
14}

Using the promise returned from the execution function

The execution function returns a promise that resolves with the query result:

1function GetDogs() {
2  const [getDogs, { data, loading, error }] = useLazyQuery(GET_DOGS);
3
4  const handleClick = async () => {
5    const { data } = await getDogs();
6
7    // Do something with `data`
8  };
9
10  return (
11    <div>
12      {/* ... */}
13      <button onClick={handleClick}>Get dogs</button>
14    </div>
15  );
16}

The result returned from the promise is useful when you need to execute side-effects using the data returned by the query.

1import { useApolloClient } from "@apollo/client/react";
2
3function GetDogs() {
4  const client = useApolloClient();
5
6  const handleClick = async () => {
7    const { data } = await client.query({ query: GET_DOGS });
8
9    // Do something with `data`
10  };
11
12  return <button onClick={handleClick}>Get dogs</button>;
13}

Handling errors

The promise resolves or rejects depending on the configured errorPolicy. For a more comprehensive guide on working with errors, read the Error handling documentation.

errorPolicy: "none"

The promise rejects with the error that caused the query to fail.

1const handleClick = async () => {
2  try {
3    const { data } = await getDogs();
4  } catch (error) {
5    if (CombinedGraphQLErrors.is(error)) {
6      // handle GraphQL errors
7    }
8
9    // Handle other error types
10    console.log(error.message);
11  }
12};

errorPolicy: "all"

The promise resolves with an object that includes the error and any partial data returned by the query. Read the partial data on the data property and the error that caused the query to fail on the error property.

1const handleClick = async () => {
2  const { data, error } = await getDogs();
3
4  if (error && CombinedGraphQLErrors.is(error)) {
5    // handle GraphQL errors returned by the query
6  }
7};

errorPolicy: "ignore"

The promise resolves with an object that includes any partial data returned by the query. Errors are discarded.

1const handleClick = async () => {
2  const { data } = await getDogs();
3
4  if (data !== undefined) {
5    // Do something with the returned data
6  }
7};

Retaining query results

Apollo Client aborts in-flight queries executed by useLazyQuery when the component unmounts or when you start another query by calling the execution function. This causes the promise to reject. In some cases, you might find this behavior undesirable and prefer to let the query run to completion.

The promise returned by the execution function includes a .retain() method. When you call it, it ensures the query continues running even if the component unmounts or you start a new query before the previous one finishes.

1function GetDogs() {
2  const [getDogs] = useLazyQuery(GET_DOGS);
3
4  const handleClick = async () => {
5    const promise = getDogs();
6
7    // Retain the query even if component unmounts
8    promise.retain();
9
10    const { data } = await promise;
11
12    // Do something with data
13  };
14
15  return <button onClick={handleClick}>Get dogs</button>;
16}

The retain() method returns the original promise, so you can shorten the previous example to a single line:

1const { data } = await getDogs().retain();

For a complete list of supported options and result properties, see the useLazyQuery API reference.

Setting a fetch policy

By default, the useQuery hook checks the Apollo Client cache to see if all the data you requested is already available. If all of the data is available in the cache, useQuery returns that data and doesn't query your GraphQL server. This cache-first policy is Apollo Client's default fetch policy.

You specify a fetch policy using the fetchPolicy option. For example, we can instruct useQuery to bypass the cache and fetch from the network by setting the fetchPolicy option to network-only:

1const { loading, error, data } = useQuery(GET_DOGS, {
2  fetchPolicy: "network-only", // Doesn't check cache before making a network request
3});

nextFetchPolicy

You can also specify a query's nextFetchPolicy. When provided, fetchPolicy is used for the query's first execution, and nextFetchPolicy is used to determine how the query responds to future cache updates:

1const { loading, error, data } = useQuery(GET_DOGS, {
2  fetchPolicy: "network-only", // Used for first execution
3  nextFetchPolicy: "cache-first", // Used for subsequent executions
4});

For example, this is helpful if you want a query to always make an initial network request, but you're comfortable reading from the cache after that.

nextFetchPolicy functions

If you want to apply a single nextFetchPolicy by default, because you find yourself manually providing nextFetchPolicy for most of your queries, you can configure defaultOptions.watchQuery.nextFetchPolicy when creating your ApolloClient instance:

1new ApolloClient({
2  link,
3  client,
4  defaultOptions: {
5    watchQuery: {
6      nextFetchPolicy: "cache-only",
7    },
8  },
9});

This configuration applies to all client.watchQuery calls and useQuery calls that do not otherwise configure nextFetchPolicy.

If you want more control over how nextFetchPolicy behaves, you can provide a function instead of a WatchQueryFetchPolicy string:

1new ApolloClient({
2  link,
3  client,
4  defaultOptions: {
5    watchQuery: {
6      nextFetchPolicy(currentFetchPolicy) {
7        if (
8          currentFetchPolicy === "network-only" ||
9          currentFetchPolicy === "cache-and-network"
10        ) {
11          // Demote the network policies (except "no-cache") to "cache-first"
12          // after the first request.
13          return "cache-first";
14        }
15        // Leave all other fetch policies unchanged.
16        return currentFetchPolicy;
17      },
18    },
19  },
20});

This nextFetchPolicy function will be called after each request, and uses the currentFetchPolicy parameter to decide how to modify the fetch policy.

In addition to being called after each request, your nextFetchPolicy function will also be called when variables change, which by default resets the fetchPolicy to its initial value, which is often important to trigger a fresh network request for queries that started out with cache-and-network or network-only fetch policies.

To intercept and handle the variables-changed case yourself, you can use the NextFetchPolicyContext object passed as the second argument to your nextFetchPolicy function:

1new ApolloClient({
2  link,
3  client,
4  defaultOptions: {
5    watchQuery: {
6      nextFetchPolicy(
7        currentFetchPolicy,
8        {
9          // Either "after-fetch" or "variables-changed", indicating why the
10          // nextFetchPolicy function was invoked.
11          reason,
12          // The rest of the options (currentFetchPolicy === options.fetchPolicy).
13          options,
14          // The original value of options.fetchPolicy, before nextFetchPolicy was
15          // applied for the first time.
16          initialFetchPolicy,
17          // The ObservableQuery associated with this client.watchQuery call.
18          observable,
19        }
20      ) {
21        // When variables change, the default behavior is to reset
22        // options.fetchPolicy to context.initialFetchPolicy. If you omit this logic,
23        // your nextFetchPolicy function can override this default behavior to
24        // prevent options.fetchPolicy from changing in this case.
25        if (reason === "variables-changed") {
26          return initialFetchPolicy;
27        }
28
29        if (
30          currentFetchPolicy === "network-only" ||
31          currentFetchPolicy === "cache-and-network"
32        ) {
33          // Demote the network policies (except "no-cache") to "cache-first"
34          // after the first request.
35          return "cache-first";
36        }
37
38        // Leave all other fetch policies unchanged.
39        return currentFetchPolicy;
40      },
41    },
42  },
43});

In order to debug these nextFetchPolicy transitions, it can be useful to add console.log or debugger statements to the function body, to see when and why the function is called.

Supported fetch policies

useQuery API

Supported options and result fields for the useQuery hook are listed below.

Most calls to useQuery can omit the majority of these options, but it's useful to know they exist. To learn about the useQuery hook API in more detail with usage examples, see the API reference.

Options

The useQuery hook accepts the following options:

Result

After being called, the useQuery hook returns a result object with the following properties. This object contains your query result, plus some helpful functions for refetching, dynamic polling, and pagination.

Next steps

Now that you understand how to fetch data with the useQuery hook, learn how to update your data with the useMutation hook!

After that, learn about some other handy Apollo Client features:

• Local state management: Learn how to query local data.

• Pagination: Learn how to fetch data incrementally from list fields.