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

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

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.

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 });

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

paulgv/mock-apollo-client