Mock Apollo Client

Helps unit test components which use the Apollo Client.

Versions

Version 0.x of this library is compatible with Apollo client 2. View the README for 0.x here.

Version 1.x of this library is compatible with Apollo client 3 (this README)

Motivation

Whilst using the impressive @apollo/client library, I ran into issues while trying to unit test components which used the GraphQL Query and Mutation components. 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, enzyme and Jest, 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 and are written in TypeScript.

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.tsx

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

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

export const Dog: React.FunctionComponent<{ name: string }> = ({ name }) => {
  const { loading, error, data } = useQuery(
    GET_DOG_QUERY,
    { variables: { name } }
  );
  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error!</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.tsx

import { ApolloProvider } from '@apollo/client';
import { mount, ReactWrapper } from 'enzyme';
import { createMockClient } from 'mock-apollo-client';
import * as React from 'react';

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

let wrapper: ReactWrapper;

beforeEach(() => {
  const mockClient = createMockClient();

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

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

it('renders the dog name and breed', () => {
  expect(wrapper.text()).toContain('Rufus is a Poodle');
});

This test file does the following:

Instantiates a new mock Apollo client
Calls setRequestHandler on the mock Apollo client to set a function to be called when the Apollo client executes the Dog query
Uses the mock and initialises the enzyme wrapper for the unit tests

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).toBeCalledTimes(1);
  expect(queryHandler).toBeCalledWith({ 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 GraphQL network error, the request handler should return a rejected promise. i.e.

mockApolloClient.setRequestHandler(
  GET_DOG_QUERY,
  () => Promise.reject(new Error('GraphQL 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.tsx

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

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

export const DogSubscription: React.FunctionComponent<{ name: string }> = ({ name }) => {
  const { loading, error, data } = useSubscription(
    SUBSCRIBE_DOG_DOCUMENT,
    { 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.tsx

import { ApolloProvider } from '@apollo/client';
import { mount, ReactWrapper } from 'enzyme';
import { createMockClient, createMockSubscription, IMockSubscription } from 'mock-apollo-client';
import { act } from 'react-dom/test-utils';
import * as React from 'react';

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

let wrapper: ReactWrapper;
let mockSubscription: IMockSubscription;

beforeEach(() => {
  const mockClient = createMockClient();
  mockSubscription = createMockSubscription();

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

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

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

  expect(wrapper.text()).toContain('Rufus has barked 0 time(s)');

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

  expect(wrapper.text()).toContain('Rufus has barked 1 time(s)');
});

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('GraphQL 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: IMockSubscription[] = [];

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 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 });

Additionally, you can specify a missingHandlerPolicy to define the behavior of the mock client when a request handler for a particular operation is not found.

The missingHandlerPolicy accepts one of three string values:

'throw-error': The client throws an error when it encounters a missing handler.
'warn-and-return-error': The client logs a warning message in the console and returns an error.
'return-error': The client returns an error without any warning message.

Here's an example of how you can set the missingHandlerPolicy:

const mockClient = createMockClient({ missingHandlerPolicy: 'warn-and-return-error' });

In this example, if a request handler for a given operation is not found, the client will log a warning message to the console and then return an error.

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

Fragments

If your 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 });

Mike-Gibson/mock-apollo-client