|
1 | 1 | # Fetch |
| 2 | + |
| 3 | +## Introduction |
| 4 | + |
| 5 | +React Native provides the Fetch API for your networking needs. It is promise-based and provides a |
| 6 | +simple and clean API for making requests. In this guide, we will show you how to mock `fetch` requests |
| 7 | +and guard your test suits from unwanted and unmocked API requests. |
| 8 | + |
| 9 | +:::info |
| 10 | +To simulate a real-world scenario, we will use |
| 11 | +the [Random User Generator API](https://randomuser.me/) |
| 12 | +that provides random user data. |
| 13 | +::: |
| 14 | + |
| 15 | +## Phonebook Example |
| 16 | + |
| 17 | +Let's assume we have a simple phonebook application that uses `fetch` for fetching Data from a server. |
| 18 | +In our case, we have a list of contacts that we want to display in our application. |
| 19 | + |
| 20 | +This is how the root of the application looks like: |
| 21 | + |
| 22 | +```tsx title=network-requests/Phonebook.tsx |
| 23 | +import React, {useEffect, useState} from 'react'; |
| 24 | +import {User} from './types'; |
| 25 | +import FavoritesList from './components/FavoritesList'; |
| 26 | + |
| 27 | +export default () => { |
| 28 | + const [usersData, setUsersData] = useState<User[]>([]); |
| 29 | + const [error, setError] = useState<string | null>(null); |
| 30 | + |
| 31 | + useEffect(() => { |
| 32 | + const run = async () => { |
| 33 | + try { |
| 34 | + const _data = await getAllContacts(); |
| 35 | + setUsersData(_data); |
| 36 | + } catch (e) { |
| 37 | + setError(e.message); |
| 38 | + } |
| 39 | + }; |
| 40 | + |
| 41 | + void run(); |
| 42 | + }, []); |
| 43 | + |
| 44 | + if (error) { |
| 45 | + return <Text>An error occurred: {error}</Text>; |
| 46 | + } |
| 47 | + |
| 48 | + return ( |
| 49 | + <ContactsList users={usersData}/> |
| 50 | + ); |
| 51 | +}; |
| 52 | + |
| 53 | +``` |
| 54 | + |
| 55 | +We fetch the contacts from the server using the `getAllContacts` function that utilizes `fetch`. |
| 56 | + |
| 57 | +```tsx title=network-requests/api/getAllContacts.ts |
| 58 | +import {User} from '../types'; |
| 59 | + |
| 60 | +export default async (): Promise<User[]> => { |
| 61 | + const res = await fetch('https://randomuser.me/api/?results=25'); |
| 62 | + if (!res.ok) { |
| 63 | + throw new Error(`Error fetching contacts`); |
| 64 | + } |
| 65 | + const json = await res.json(); |
| 66 | + return json.results; |
| 67 | +}; |
| 68 | +``` |
| 69 | + |
| 70 | +Our `ContactsList` component is a simple component that displays the list of favorite contacts and |
| 71 | +their avatars. |
| 72 | + |
| 73 | +```tsx title=network-requests/components/ContactsList.tsx |
| 74 | +import {FlatList, Image, StyleSheet, Text, View} from 'react-native'; |
| 75 | +import React, {useCallback} from 'react'; |
| 76 | +import type {ListRenderItem} from '@react-native/virtualized-lists'; |
| 77 | +import {User} from '../types'; |
| 78 | + |
| 79 | +export default ({users}: { users: User[] }) => { |
| 80 | + const renderItem: ListRenderItem<User> = useCallback( |
| 81 | + ({item: {name, email, picture, cell}, index}) => { |
| 82 | + const {title, first, last} = name; |
| 83 | + const backgroundColor = index % 2 === 0 ? '#f9f9f9' : '#fff'; |
| 84 | + return ( |
| 85 | + <View style={[{backgroundColor}, styles.userContainer]}> |
| 86 | + <Image source={{uri: picture.thumbnail}} style={styles.userImage}/> |
| 87 | + <View> |
| 88 | + <Text> |
| 89 | + Name: {title} {first} {last} |
| 90 | + </Text> |
| 91 | + <Text>Email: {email}</Text> |
| 92 | + <Text>Mobile: {cell}</Text> |
| 93 | + </View> |
| 94 | + </View> |
| 95 | + ); |
| 96 | + }, |
| 97 | + [], |
| 98 | + ); |
| 99 | + |
| 100 | + if (users.length === 0) return ( |
| 101 | + <View style={styles.loaderContainer}> |
| 102 | + <Text>Users data not quite there yet...</Text> |
| 103 | + </View> |
| 104 | + ); |
| 105 | + |
| 106 | + return ( |
| 107 | + <View> |
| 108 | + <FlatList<User> |
| 109 | + data={users} |
| 110 | + renderItem={renderItem} |
| 111 | + keyExtractor={(item, index) => `${index}-${item.id.value}`} |
| 112 | + /> |
| 113 | + </View> |
| 114 | + ); |
| 115 | +}; |
| 116 | + |
| 117 | +// Looking for styles? |
| 118 | +// Check examples/cookbook/app/network-requests/components/ContactsList.tsx |
| 119 | +const styles = |
| 120 | +... |
| 121 | +``` |
| 122 | + |
| 123 | +## Start testing with a simple test |
| 124 | + |
| 125 | +In our test we will make sure we mock the `fetch` function to return the data we want. |
| 126 | +In this specific case, we will return a list of 3 users. |
| 127 | +As the `fetch` api is available globally, we can mock it by using `jest.spyOn` specifically on the |
| 128 | +`global` object. |
| 129 | + |
| 130 | +:::info |
| 131 | +By using `mockResolvedValueOnce` we gain more grip and prevent the mock from resolving the data |
| 132 | +multiple times, which might lead to unexpected behavior. |
| 133 | +::: |
| 134 | + |
| 135 | +```tsx title=network-requests/Phonebook.test.tsx |
| 136 | +import {render, waitForElementToBeRemoved} from '@testing-library/react-native'; |
| 137 | +import React from 'react'; |
| 138 | +import PhoneBook from '../PhoneBook'; |
| 139 | +import {User} from '../types'; |
| 140 | + |
| 141 | +describe('PhoneBook', () => { |
| 142 | + it('fetches contacts successfully and renders in list', async () => { |
| 143 | + // mock the fetch function to return the data we want |
| 144 | + jest.spyOn(global, 'fetch').mockResolvedValueOnce({ |
| 145 | + json: jest.fn().mockResolvedValueOnce(DATA), |
| 146 | + }); |
| 147 | + render(<PhoneBook/>); |
| 148 | + |
| 149 | + // Wait for the loader to disappear |
| 150 | + await waitForElementToBeRemoved(() => screen.getByText(/users data not quite there yet/i)); |
| 151 | + // Check if the users are displayed |
| 152 | + expect(await screen.findByText('Name: Mrs Ida Kristensen')).toBeOnTheScreen(); |
| 153 | + expect( await screen. findByText( 'Email: [email protected]')). toBeOnTheScreen(); |
| 154 | + expect(await screen.findAllByText(/name/i)).toHaveLength(3); |
| 155 | + }); |
| 156 | +}); |
| 157 | + |
| 158 | +const DATA: { results: User[] } = { |
| 159 | + results: [ |
| 160 | + { |
| 161 | + name: { |
| 162 | + title: 'Mrs', |
| 163 | + first: 'Ida', |
| 164 | + last: 'Kristensen', |
| 165 | + }, |
| 166 | + |
| 167 | + id: { |
| 168 | + name: 'CPR', |
| 169 | + value: '250562-5730', |
| 170 | + }, |
| 171 | + picture: { |
| 172 | + large: 'https://randomuser.me/api/portraits/women/26.jpg', |
| 173 | + medium: 'https://randomuser.me/api/portraits/med/women/26.jpg', |
| 174 | + thumbnail: 'https://randomuser.me/api/portraits/thumb/women/26.jpg', |
| 175 | + }, |
| 176 | + cell: '123-4567-890', |
| 177 | + }, |
| 178 | + // For brevity, we have omitted the rest of the users, you can still find them in |
| 179 | + // examples/cookbook/app/network-requests/__tests__/PhoneBook.test.tsx |
| 180 | + ... |
| 181 | + ], |
| 182 | +}; |
| 183 | + |
| 184 | +``` |
| 185 | + |
| 186 | +## Testing error handling |
| 187 | + |
| 188 | +As we are dealing with network requests, we should also test how our application behaves when the |
| 189 | +API request fails. We can mock the `fetch` function to throw an error and/or mark it's response as |
| 190 | +not 'ok' in order to verify if our application is handling the error correctly. |
| 191 | + |
| 192 | +:::note |
| 193 | +The `fetch` function will reject the promise on some errors, but not if the server responds |
| 194 | +with an error status like 404: so we also check the response status and throw if it is not OK. |
| 195 | +See MDN's [docs](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch) for more |
| 196 | +::: |
| 197 | + |
| 198 | +```tsx title=network-requests/Phonebook.test.tsx |
| 199 | +... |
| 200 | +it('fails to fetch contacts and renders error message', async () => { |
| 201 | + // mock the fetch function to be not ok which will throw an error |
| 202 | + (global.fetch as jest.SpyInstance).mockResolvedValueOnce({ |
| 203 | + ok: false, |
| 204 | + }); |
| 205 | + render(<PhoneBook/>); |
| 206 | + |
| 207 | + // Wait for the loader to disappear |
| 208 | + await waitForElementToBeRemoved(() => screen.getByText(/users data not quite there yet/i)); |
| 209 | + // Check if the error message is displayed |
| 210 | + expect(await screen.findByText(/error fetching contacts/i)).toBeOnTheScreen(); |
| 211 | +}); |
| 212 | +```` |
| 213 | + |
| 214 | +## Global guarding against unwanted API requests |
| 215 | + |
| 216 | +As mistakes may happen, we might forget to mock an API request in one of our tests in the future. |
| 217 | +To prevent we make unwanted API requests, and alert the developer when it happens, we can globally |
| 218 | +mock the `fetch` in our test suite via the `jest.setup.ts` file. Ensure this setup file is included |
| 219 | +in [`setupFilesAfterEnv`](https://jestjs.io/docs/configuration#setupfilesafterenv-array) in your Jest configuration. |
| 220 | + |
| 221 | +```tsx title=jest.setup.ts |
| 222 | +beforeAll(() => { |
| 223 | + // the global fetch function: |
| 224 | + jest.spyOn(global, 'fetch').mockImplementation(() => { |
| 225 | + throw Error("Please ensure you mock 'fetch' Only Chuck Norris is allowed to make API requests when testing ;)"); |
| 226 | + }); |
| 227 | +}); |
| 228 | +afterAll(() => { |
| 229 | + // restore the original fetch function |
| 230 | + (global.fetch as jest.Mock).mockRestore(); |
| 231 | +}); |
| 232 | +
|
| 233 | +``` |
| 234 | + |
| 235 | +## Conclusion |
| 236 | + |
| 237 | +Testing a component that makes network requests with `fetch` is straightforward. By mocking the fetch |
| 238 | +requests, we can control the data that is returned and test how our application behaves in different |
| 239 | +scenarios, such as when the request is successful or when it fails. |
| 240 | +There are many ways to mock `fetch` requests, and the method you choose will depend on your specific |
| 241 | +use case. In this guide, we showed you how to mock `fetch` requests using Jest's `jest.spyOn` function |
| 242 | +and how to guard against unwanted API requests throughout your test suite. |
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