From v27 to v28

Upgrading Jest from v27 to v28? This guide aims to help refactoring your configuration and tests.

info

See changelog for the full list of changes.

Compatibility

The supported Node versions are 12.13, 14.15, 16.10 and above.

If you plan to use type definitions of Jest (or any of its packages), make sure to install TypeScript version 4.3 or above.

Configuration Options

extraGlobals

The extraGlobals option was renamed to sandboxInjectedGlobals:

- extraGlobals: ['Math']
+ sandboxInjectedGlobals: ['Math']

timers

The timers option was renamed to fakeTimers. See Fake Timers section below for details.

testURL

The testURL option is removed. Now you should use testEnvironmentOptions to pass url option to JSDOM environment:

- testURL: 'https://jestjs.io'
+ testEnvironmentOptions: {
+   url: 'https://jestjs.io'
+ }

Babel config

babel-jest now passes root: config.rootDir to Babel when resolving configuration. This improves compatibility when using projects with differing configuration, but it might mean your babel config isn’t picked up in the same way anymore. You can override this option by passing options to babel-jest in your configuration.

expect

In versions prior to Jest 28, toHaveProperty checked for equality instead of existence, which means that e.g. expect({}).toHaveProperty('a', undefined) is a passing test. This has been changed in Jest 28 to fail.

Additionally, if you import expect directly, it has been changed from default export to a named export.

- import expect from 'expect';
+ import {expect} from 'expect';

- const expect = require('expect');
+ const {expect} = require('expect');

假的定时器

Fake timers were refactored to allow passing options to the underlying @sinonjs/fake-timers.

fakeTimers

The timers configuration option was renamed to fakeTimers and now takes an object with options:

- timers: 'real'
+ fakeTimers: {
+   enableGlobally: false
+ }

- timers: 'fake'
+ fakeTimers: {
+   enableGlobally: true
+ }

- timers: 'modern'
+ fakeTimers: {
+   enableGlobally: true
+ }

- timers: 'legacy'
+ fakeTimers: {
+   enableGlobally: true,
+   legacyFakeTimers: true
+ }

jest.useFakeTimers()

An object with options now should be passed to jest.useFakeTimers() as well:

- jest.useFakeTimers('modern')
+ jest.useFakeTimers()

- jest.useFakeTimers('legacy')
+ jest.useFakeTimers({
+   legacyFakeTimers: true
+ })

If legacy fake timers are enabled in Jest config file, but you would like to disable them in a particular test file:

- jest.useFakeTimers('modern')
+ jest.useFakeTimers({
+   legacyFakeTimers: false
+ })

Test Environment

Custom Environment

The constructor of test environment class now receives an object with Jest’s globalConfig and projectConfig as its first argument. The second argument is now mandatory.

  class CustomEnvironment extends NodeEnvironment {
-   constructor(config) {
-     super(config);
+   constructor({globalConfig, projectConfig}, context) {
+     super({globalConfig, projectConfig}, context);
+     const config = projectConfig;

In addition, test environments are now exported with the name TestEnvironment, instead of simply exporting the class directly:

- const TestEnvironment = require('jest-environment-node');
+ const {TestEnvironment} = require('jest-environment-node');
- const TestEnvironment = require('jest-environment-jsdom');
+ const {TestEnvironment} = require('jest-environment-jsdom');

jsdom

If you are using JSDOM test environment, jest-environment-jsdom package now must be installed separately:

• npm
• Yarn

npm install --save-dev jest-environment-jsdom

yarn add --dev jest-environment-jsdom

Test Runner

If you are using Jasmine test runner, jest-jasmine2 package now must be installed separately:

• npm
• Yarn

npm install --save-dev jest-jasmine2

yarn add --dev jest-jasmine2

Transformer

process() and processAsync() methods of a custom transformer module cannot return a string anymore. They must always return an object:

  process(sourceText, sourcePath, options) {
-   return `module.exports = ${JSON.stringify(path.basename(sourcePath))};`;
+   return {
+     code: `module.exports = ${JSON.stringify(path.basename(sourcePath))};`,
+   };
  }

package.json exports

Jest now includes full support for package exports, which might mean that files you import are not resolved correctly.

Additionally, Jest now supplies more conditions. jest-environment-node has node and node-addons, while jest-environment-jsdom has browser. As a result, you might e.g. get browser code which assumes ESM, when Jest provides ['require', 'browser']. You can either report a bug to the library (or Jest, the implementation is new and might have bugs!), override the conditions Jest passes (by passing the customExportConditions option to the test environment), or use a custom resolver or moduleMapper. Lots of options, and you’ll need to pick the correct one for your project.

Known examples of packages that fails in Jest 28 are uuid and nanoid when using the jest-environment-jsdom environment. For an analysis, and a potential workaround, see this comment.

TypeScript

info

The TypeScript examples from this page will only work as documented if you import jest from '@jest/globals':

import {jest} from '@jest/globals';

jest.fn()

jest.fn() now takes only one generic type argument. See Mock Functions API page for more usage examples.

  import add from './add';
- const mockAdd = jest.fn<ReturnType<typeof add>, Parameters<typeof add>>();
+ const mockAdd = jest.fn<typeof add>();

- const mock = jest.fn<number, []>()
+ const mock = jest.fn<() => number>()
    .mockReturnValue(42)
    .mockReturnValueOnce(12);
- const asyncMock = jest.fn<Promise<string>, []>()
+ const asyncMock = jest.fn<() => Promise<string>>()
    .mockResolvedValue('default')
    .mockResolvedValueOnce('first call');