Introduction

GraphQL has become a popular choice for managing data in modern web applications, especially in React projects. Among the tools provided by Apollo, a popular GraphQL client, are useQuery and useLazyQuery hooks. This blog aims to provide a clear and simple understanding of these hooks, highlighting their differences and use cases, which is particularly useful for developers working in the tech industry.

What is useQuery?

useQuery is a hook provided by Apollo Client to fetch data from a GraphQL server. It is used to execute a query when a component renders.

Characteristics of useQuery:

• Automatic Execution: The query runs automatically when the component mounts.

• Reactive: The query re-runs when variables change.

• Control Over Fetching: Offers options like skip, poll, and refetch.

Example of useQuery:

import { useQuery, gql } from '@apollo/client';

const GET_DOGS = gql`
  query GetDogs {
    dogs {
      id
      breed
    }
  }
`;

function Dogs() {
  const { loading, error, data } = useQuery(GET_DOGS);

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error :(</p>;

  return data.dogs.map(dog => <Dog key={dog.id} dog={dog} />);
}

In this example, the GET_DOGS query is executed as soon as the Dogs component renders.

What is useLazyQuery?

useLazyQuery is another hook provided by Apollo Client, designed to execute queries in response to an event, not automatically upon component rendering.

Characteristics of useLazyQuery:

• Manual Execution: The query is triggered manually, typically in response to user interaction.

• Ideal for On-Demand Data Fetching: Use when the data is needed after certain user actions like clicking a button.

• Similar API to useQuery: Provides loading, error, and data in a similar way to useQuery.

Example of useLazyQuery:

import { useLazyQuery, gql } from '@apollo/client';

const GET_DOG_PHOTO = gql`
  query DogPhoto($breed: String!) {
    dog(breed: $breed) {
      id
      displayImage
    }
  }
`;

function DogPhoto({ breed }) {
  const [getDogPhoto, { loading, data }] = useLazyQuery(GET_DOG_PHOTO);

  if (loading) return <p>Loading...</p>;

  return (
    <div>
      <button onClick={() => getDogPhoto({ variables: { breed } })}>
        Load Photo
      </button>
      {data && <img src={data.dog.displayImage} alt="Dog" />}
    </div>
  );
}

In this example, the GET_DOG_PHOTO query is executed only when the button is clicked.

The useLazyQuery Configuration object.

When using Apollo Client's useLazyQuery hooks in a React application, you have the option to pass a configuration object. This object allows you to specify various settings that control the behavior of the query. Let's break down some of the key options you can include in this configuration object:

1. Variables

• Purpose: Used to pass dynamic values to the query.

• Usage: Essential when your GraphQL query requires input parameters.

Example:

const GET_DOG_BY_BREED = gql`
  query GetDogByBreed($breed: String!) {
    dog(breed: $breed) {
      id
      name
    }
  }
`;

const { data } = useQuery(GET_DOG_BY_BREED, {
  variables: { breed: 'bulldog' },
});

Here, the breed variable is passed to the query to fetch data for a specific dog breed.

2. Context

• Purpose: Pass additional information to the Apollo Link chain.

• Usage: Useful for setting headers, like authentication tokens, or other context-specific data.

Example:

const { data } = useQuery(GET_DOG_BY_BREED, {
  context: {
    headers: {
      Authorization: 'Bearer your-auth-token',
    },
  },
});

In this case, an authorization header is added to the query request.

3. Fetch Policy

• Purpose: Determines how the query interacts with the cache.

• Usage: Can be set to values like cache-first, network-only, cache-and-network, etc., to control the caching behavior.

Example:

const { data } = useQuery(GET_DOG_BY_BREED, {
  fetchPolicy: 'network-only',
});

This setting ensures the data is always fetched from the network, ignoring the cache.

4. Error Policy

• Purpose: Dictates the handling of query errors.

• Usage: Can be set to none, ignore, or all, influencing how errors are reported.

Example:

const { data } = useQuery(GET_DOG_BY_BREED, {
  errorPolicy: 'all',
});

This setting will include all errors in the response, allowing you to handle them as needed.

5. Poll Interval

• Purpose: Automatically refetches the query at specified intervals.

• Usage: Useful for keeping the data up-to-date.

Example:

const { data } = useQuery(GET_DOG_BY_BREED, {
  pollInterval: 5000,
});

This configuration will refetch the query every 5000 milliseconds (5 seconds).

When to Use Each?

• UseuseQuery when you need data as soon as a component renders or when the data should update reactively with component updates.

• UseuseLazyQuery for scenarios where data fetching should happen on specific events like button clicks, form submissions, or other interactive behaviors.

Conclusion

Both useQuery and useLazyQuery are essential tools in a React developer's arsenal when working with GraphQL. Understanding when and how to use each can greatly enhance the efficiency and user experience of your web applications. As you aspire to become a senior software developer, mastering such tools and techniques is crucial in building sophisticated and responsive applications.