Mock Apollo Client

Helps unit test components which use the Apollo Client.

Versions

Version	Compatibility
0.x	Apollo client 2. README for 0.x here.
1.x	Apollo client 3. README for 1.x here.
2.x	Apollo client 4. (this README)

Motivation

When using the @apollo/client library, I ran into issues trying to unit test React components which called GraphQL queries and mutations.

The Apollo client library includes a MockedProvider component which allows query and mutation results to be mocked, but didn't offer enough control within unit tests. The Apollo client documentation for testing can be found here.

Specifically, some of the issues I faced were:

• Unable to assert queries/mutations were called with the expected variables
• Unable to assert how many times a specific query was called
• Unable to change the query/mutation result after the MockedProvider was initialised
• Unable to easily control the query/mutation loading state

The mock-apollo-client library helps with the above issues, by allowing more control within unit tests.

Installation

npm install --save-dev mock-apollo-client

Usage

The examples below use React, Jest and React Testing Library, but mock-apollo-client is standalone and can used with any libraries and test frameworks.

The examples have been adapted from the official Apollo testing docs where possible.

Simple Query Example

Consider the file below, which contains a single GraphQL query and a component which is responsible for rendering the result of the query:

// dog.jsx

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

export const GET_DOG_QUERY = gql`
  query getDog($name: String) {
    dog(name: $name) {
      id
      name
      breed
    }
  }
`;

export const Dog = ({ name }) => {
  const { loading, error, data } = useQuery(GET_DOG_QUERY, {
    variables: { name },
  });
  if (loading) return <p>Loading...</p>;
  if (error) return <p>{error.message}</p>;

  return (
    <p>
      {data.dog.name} is a {data.dog.breed}
    </p>
  );
};

To unit test this component using mock-apollo-client, the test file could look like the following:

// dog.test.jsx
import '@testing-library/jest-dom';
import { ApolloProvider } from '@apollo/client/react';
import { render, screen } from '@testing-library/react';
import { createMockClient } from 'mock-apollo-client';

import { GET_DOG_QUERY, Dog } from './dog';

it('renders the dog name and breed', async () => {
  const mockClient = createMockClient();

  mockClient.setRequestHandler(GET_DOG_QUERY, () =>
    Promise.resolve({
      data: { dog: { id: 1, name: 'Rufus', breed: 'Poodle' } },
    }),
  );

  render(
    <ApolloProvider client={mockClient}>
      <Dog name="Rufus" />
    </ApolloProvider>,
  );

  expect(await screen.findByText('Rufus is a Poodle')).toBeInTheDocument();
});

This test file does the following:

1. Instantiates a new mock Apollo client
2. Calls setRequestHandler on the mock Apollo client instance to set a function to be called when Apollo client executes the Dog query
3. Passes the mock Apollo client instance to the ApolloProvider when rendering the component

Asserting query variables

The method setRequestHandler is passed a function to call when Apollo client executes a given query and it is called with the variables for that query, so it is easy to assert the component is behaving as expected using a spy library.

const queryHandler = jest.fn().mockResolvedValue({
  data: { dog: { id: 1, name: 'Rufus', breed: 'Poodle' } },
});

mockApolloClient.setRequestHandler(GET_DOG_QUERY, queryHandler);

// ....

it('executes the query with the correct variables', () => {
  expect(queryHandler).toHaveBeenCalledTimes(1);
  expect(queryHandler).toHaveBeenCalledWith({ name: 'Rufus' });
});

Loading states

A request handler returns a promise, so testing for loading state just requires that the promise returned is not resolved or rejected.

Error states

To simulate a network error, the request handler should return a rejected promise. i.e.

mockApolloClient.setRequestHandler(GET_DOG_QUERY, () =>
  Promise.reject(new Error('Network Error')),
);

To simulate GraphQL errors, the request handler should return a Promise which resolves with an errors field. i.e.

mockApolloClient.setRequestHandler(GET_DOG_QUERY, () =>
  Promise.resolve({ errors: [{ message: 'GraphQL Error' }] }),
);

Mutations

Mutations can be tested the same way that queries are, by using setRequestHandler and specifying a request handler for the mutation query.

Subscriptions

Subscriptions can be tested, but require a different setup as they receive a stream of data. Consider the file below, which contains a single subscription and a component which is responsible for rendering the updated data:

// dogSubscription.jsx

import { gql } from '@apollo/client';
import { useSubscription } from '@apollo/client/react';

export const DOG_SUBSCRIPTION = gql`
  subscription subscribeDog($name: String) {
    dog(name: $name) {
      id
      name
      numberOfBarks
    }
  }
`;

export const DogSubscription = ({ name }) => {
  const { loading, error, data } = useSubscription(DOG_SUBSCRIPTION, {
    variables: { name },
  });
  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error!</p>;

  return (
    <p>
      {data.dog.name} has barked {data.dog.numberOfBarks} time(s)
    </p>
  );
};

To unit test this component using mock-apollo-client, the test file could look like the following:

// dogSubscription.test.jsx

import '@testing-library/jest-dom';
import { act } from 'react';
import { ApolloProvider } from '@apollo/client/react';
import { render, screen } from '@testing-library/react';
import { createMockClient, createMockSubscription } from 'mock-apollo-client';

import { DOG_SUBSCRIPTION, DogSubscription } from './dogSubscription';

it('renders the dog details', async () => {
  const mockClient = createMockClient();
  const mockSubscription = createMockSubscription();

  mockClient.setRequestHandler(DOG_SUBSCRIPTION, () => mockSubscription);

  render(
    <ApolloProvider client={mockClient}>
      <DogSubscription name="Rufus" />
    </ApolloProvider>,
  );

  act(() => {
    mockSubscription.next({
      data: { dog: { id: 1, name: 'Rufus', numberOfBarks: 0 } },
    });
  });

  expect(
    await screen.findByText('Rufus has barked 0 time(s)'),
  ).toBeInTheDocument();

  act(() => {
    mockSubscription.next({
      data: { dog: { id: 1, name: 'Rufus', numberOfBarks: 1 } },
    });
  });

  expect(
    await screen.findByText('Rufus has barked 1 time(s)'),
  ).toBeInTheDocument();
});

The subscription can be closed by calling .complete if necessary for the test.

Errors

You can also test error states by calling .error on the mockSubscription and passing errors as described in Error States:

mockSubscription.error(new Error('Network Error'));

Multiple subscriptions

A mock subscription will only be associated with a single invocation of a query. If a component is subscribing to the same query multiple times, then a separate mock subscription should be used for each one.

const subscriptions = [];

mockClient.setRequestHandler(
  SUBSCRIBE_DOG_DOCUMENT,
  () => {
    const subscription = createMockSubscription();
    subscriptions.push(subscription);
    return subscription;
  });

...

subscriptions.forEach((s) => s.next({ data: { dog: { id: 1, name: 'Rufus', numberOfBarks: 1 } } }));

Specifying mock Apollo client options

Apollo client options

The createMockClient method can be provided with the same constructor arguments that ApolloClient accepts which are used when instantiating the mock Apollo client.

For example, to specify the cache (and possible types for fragment matching) that should be used:

const cache = new InMemoryCache({
  possibleTypes: myPossibleTypes,
});

const mockClient = createMockClient({ cache });

Note: it is not possible to specify the link to use as this is how mock-apollo-client injects its request handling behaviour.

Mock client options

Additionally, you can specify the following options which control the behaviour of the mock Apollo client:

Option	Description	Default
suppressMissingHandlerWarning	Controls whether a warning is logged when a request handler for an executed operation is not found.	false (Warning will be logged)

Fragments

If queries or mutations use fragments against union or interface types, you must inject a cache object when creating the mock client which has been provided with possibleTypes, and also include the correct __typename field when mocking the response.

For example:

import { InMemoryCache } from '@apollo/client';
import { createMockClient } from 'mock-apollo-client';

const cache = new InMemoryCache({
  possibleTypes: {
    Hardware: ['Memory', 'Cpu'],
  },
});

const mockClient = createMockClient({ cache });

You must then ensure that the query result includes the __typename field as it would when calling your actual GraphQL API. This is to ensure that the fragment matching works as expected:

const query = gql`
  query Hardware {
    hardware {
      id
      ... on Memory {
        size
      }
      ... on Cpu {
        speed
      }
    }
  }
`;

const mockData = {
  hardware: {
    __typename: 'Memory',
    id: 2,
    size: '16gb',
  },
};

const requestHandler = jest.fn().mockResolvedValue({ data: mockData });

Removing a handler

It's possible to remove a previously registered handler for a query using removeRequestHandler.

For example:

mockApolloClient.removeRequestHandler(GET_DOG_QUERY);