eslint-plugin-jest

What is eslint-plugin-jest?

The eslint-plugin-jest package is an ESLint plugin that provides linting rules for Jest, a popular JavaScript testing framework. It helps maintain code quality and enforce best practices by analyzing test files for common issues and stylistic preferences.

What are eslint-plugin-jest's main functionalities?

Enforcing consistent test descriptions

This rule enforces a consistent test function name, either `test` or `it`, within the test files.

/* eslint jest/consistent-test-it: ["error", { fn: "test" }] */

// Incorrect
describe('myFeature', () => {
  it('does something', () => {
    // test implementation
  });
});

// Correct
describe('myFeature', () => {
  test('does something', () => {
    // test implementation
  });
});

Preventing disabled tests

This rule prevents the use of `xdescribe`, `xit`, or `test.skip`, which are used to disable tests, to ensure that all tests are run.

/* eslint jest/no-disabled-tests: "error" */

// Incorrect
xdescribe('myFeature', () => {
  test('does something', () => {});
});

xit('does something', () => {});

// Correct
describe('myFeature', () => {
  test('does something', () => {});
});

Ensuring tests contain assertions

This rule ensures that test blocks contain at least one assertion call, which is necessary for a meaningful test.

/* eslint jest/expect-expect: "error" */

// Incorrect
test('does something', () => {
  // no assertions
});

// Correct
test('does something', () => {
  expect(something).toBe(true);
});

Disallowing identical titles

This rule disallows using the same title for multiple test cases or `describe` blocks, which can cause confusion when trying to identify tests.

/* eslint jest/no-identical-title: "error" */

// Incorrect
describe('myFeature', () => {
  test('does something', () => {});
  test('does something', () => {});
});

// Correct
describe('myFeature', () => {
  test('does something', () => {});
  test('does something else', () => {});
});

Other packages similar to eslint-plugin-jest

eslint-plugin-mocha

This package provides linting rules for Mocha, another JavaScript test framework. It is similar to eslint-plugin-jest but tailored for Mocha's API and conventions.

eslint-plugin-jasmine

This package offers linting rules for Jasmine, a behavior-driven development framework for testing JavaScript code. It serves a similar purpose to eslint-plugin-jest but is specific to Jasmine's syntax and style.

eslint-plugin-testing-library

This ESLint plugin enforces best practices for the Testing Library, which is often used with Jest for testing React components. It complements eslint-plugin-jest by focusing on the specific patterns and practices of the Testing Library.

eslint-plugin-cypress

This plugin provides linting rules for Cypress, an end-to-end testing framework. While it is not specific to Jest, it offers similar functionality for maintaining code quality in a different testing context.

README

eslint-plugin-jest

ESLint plugin for Jest

Installation

yarn add --dev eslint eslint-plugin-jest

Note: If you installed ESLint globally then you must also install eslint-plugin-jest globally.

Usage

[!NOTE]

eslint.config.js is supported, though most of the plugin documentation still currently uses .eslintrc syntax.

Refer to the ESLint documentation on the new configuration file format for more.

Add jest to the plugins section of your .eslintrc configuration file. You can omit the eslint-plugin- prefix:

{
  "plugins": ["jest"]
}

Then configure the rules you want to use under the rules section.

{
  "rules": {
    "jest/no-disabled-tests": "warn",
    "jest/no-focused-tests": "error",
    "jest/no-identical-title": "error",
    "jest/prefer-to-have-length": "warn",
    "jest/valid-expect": "error"
  }
}

You can also tell ESLint about the environment variables provided by Jest by doing:

{
  "env": {
    "jest/globals": true
  }
}

This is included in all configs shared by this plugin, so can be omitted if extending them.

Aliased Jest globals

You can tell this plugin about any global Jests you have aliased using the globalAliases setting:

{
  "settings": {
    "jest": {
      "globalAliases": {
        "describe": ["context"],
        "fdescribe": ["fcontext"],
        "xdescribe": ["xcontext"]
      }
    }
  }
}

Running rules only on test-related files

The rules provided by this plugin assume that the files they are checking are test-related. This means it's generally not suitable to include them in your top-level configuration as that applies to all files being linted which can include source files.

For .eslintrc configs you can use overrides to have ESLint apply additional rules to specific files:

{
  "extends": ["eslint:recommended"],
  "overrides": [
    {
      "files": ["test/**"],
      "plugins": ["jest"],
      "extends": ["plugin:jest/recommended"],
      "rules": { "jest/prefer-expect-assertions": "off" }
    }
  ],
  "rules": {
    "indent": ["error", 2]
  }
}

For eslint.config.js you can use files and ignores:

const jest = require('eslint-plugin-jest');

module.exports = [
  ...require('@eslint/js').configs.recommended,
  {
    files: ['test/**'],
    ...jest.configs['flat/recommended'],
    rules: {
      ...jest.configs['flat/recommended'].rules,
      'jest/prefer-expect-assertions': 'off',
    },
  },
  // you can also configure jest rules in other objects, so long as some of the `files` match
  {
    files: ['test/**'],
    rules: { 'jest/prefer-expect-assertions': 'off' },
  },
];

Jest version setting

The behaviour of some rules (specifically no-deprecated-functions) change depending on the version of Jest being used.

By default, this plugin will attempt to determine to locate Jest using require.resolve, meaning it will start looking in the closest node_modules folder to the file being linted and work its way up.

Since we cache the automatically determined version, if you're linting sub-folders that have different versions of Jest, you may find that the wrong version of Jest is considered when linting. You can work around this by providing the Jest version explicitly in nested ESLint configs:

{
  "settings": {
    "jest": {
      "version": 27
    }
  }
}

To avoid hard-coding a number, you can also fetch it from the installed version of Jest if you use a JavaScript config file such as .eslintrc.js:

module.exports = {
  settings: {
    jest: {
      version: require('jest/package.json').version,
    },
  },
};

Shareable configurations

[!NOTE]

eslint.config.js compatible versions of configs are available prefixed with flat/ and may be subject to small breaking changes while ESLint v9 is being finalized.

Recommended

This plugin exports a recommended configuration that enforces good testing practices.

To enable this configuration with .eslintrc, use the extends property:

{
  "extends": ["plugin:jest/recommended"]
}

To enable this configuration with eslint.config.js, use jest.configs['flat/recommended']:

const jest = require('eslint-plugin-jest');

module.exports = [
  {
    files: [
      /* glob matching your test files */
    ],
    ...jest.configs['flat/recommended'],
  },
];

Style

This plugin also exports a configuration named style, which adds some stylistic rules, such as prefer-to-be-null, which enforces usage of toBeNull over toBe(null).

To enable this configuration use the extends property in your .eslintrc config file:

{
  "extends": ["plugin:jest/style"]
}

To enable this configuration with eslint.config.js, use jest.configs['flat/style']:

const jest = require('eslint-plugin-jest');

module.exports = [
  {
    files: [
      /* glob matching your test files */
    ],
    ...jest.configs['flat/style'],
  },
];

All

If you want to enable all rules instead of only some you can do so by adding the all configuration to your .eslintrc config file:

{
  "extends": ["plugin:jest/all"]
}

To enable this configuration with eslint.config.js, use jest.configs['flat/all']:

const jest = require('eslint-plugin-jest');

module.exports = [
  {
    files: [
      /* glob matching your test files */
    ],
    ...jest.configs['flat/all'],
  },
];

While the recommended and style configurations only change in major versions the all configuration may change in any release and is thus unsuited for installations requiring long-term consistency.

Snapshot processing

[!NOTE]

This is only relevant for eslint.config.js

This plugin provides a custom processor to allow rules to "lint" snapshot files.

For .eslintrc based configs, this is automatically enabled out of the box but must be opted into for eslint.config.js using the flat/snapshots config:

const jest = require('eslint-plugin-jest');

module.exports = [
  {
    ...jest.configs['flat/snapshots'],
    rules: {
      'jest/no-large-snapshots': ['error', { maxSize: 1 }],
    },
  },
];

Unlike other configs, this includes a files array that matches .snap files meaning you can use it directly

Rules

💼 Configurations enabled in.
⚠️ Configurations set to warn in.
✅ Set in the recommended configuration.
🎨 Set in the style configuration.
🔧 Automatically fixable by the --fix CLI option.
💡 Manually fixable by editor suggestions.

Name	Description	💼	⚠️	🔧	💡
consistent-test-it	Enforce test and it usage conventions			🔧
expect-expect	Enforce assertion to be made in a test body		✅
max-expects	Enforces a maximum number assertion calls in a test body
max-nested-describe	Enforces a maximum depth to nested describe calls
no-alias-methods	Disallow alias methods	✅	🎨	🔧
no-commented-out-tests	Disallow commented out tests		✅
no-conditional-expect	Disallow calling expect conditionally	✅
no-conditional-in-test	Disallow conditional logic in tests
no-confusing-set-timeout	Disallow confusing usages of jest.setTimeout
no-deprecated-functions	Disallow use of deprecated functions	✅		🔧
no-disabled-tests	Disallow disabled tests		✅
no-done-callback	Disallow using a callback in asynchronous tests and hooks	✅			💡
no-duplicate-hooks	Disallow duplicate setup and teardown hooks
no-export	Disallow using exports in files containing tests	✅
no-focused-tests	Disallow focused tests	✅			💡
no-hooks	Disallow setup and teardown hooks
no-identical-title	Disallow identical titles	✅
no-interpolation-in-snapshots	Disallow string interpolation inside snapshots	✅
no-jasmine-globals	Disallow Jasmine globals	✅		🔧
no-large-snapshots	Disallow large snapshots
no-mocks-import	Disallow manually importing from __mocks__	✅
no-restricted-jest-methods	Disallow specific jest. methods
no-restricted-matchers	Disallow specific matchers & modifiers
no-standalone-expect	Disallow using expect outside of it or test blocks	✅
no-test-prefixes	Require using .only and .skip over f and x	✅		🔧
no-test-return-statement	Disallow explicitly returning from tests
no-untyped-mock-factory	Disallow using jest.mock() factories without an explicit type parameter			🔧
prefer-called-with	Suggest using toBeCalledWith() or toHaveBeenCalledWith()
prefer-comparison-matcher	Suggest using the built-in comparison matchers			🔧
prefer-each	Prefer using .each rather than manual loops
prefer-equality-matcher	Suggest using the built-in equality matchers				💡
prefer-expect-assertions	Suggest using expect.assertions() OR expect.hasAssertions()				💡
prefer-expect-resolves	Prefer await expect(...).resolves over expect(await ...) syntax			🔧
prefer-hooks-in-order	Prefer having hooks in a consistent order
prefer-hooks-on-top	Suggest having hooks before any test cases
prefer-lowercase-title	Enforce lowercase test names			🔧
prefer-mock-promise-shorthand	Prefer mock resolved/rejected shorthands for promises			🔧
prefer-snapshot-hint	Prefer including a hint with external snapshots
prefer-spy-on	Suggest using jest.spyOn()			🔧
prefer-strict-equal	Suggest using toStrictEqual()				💡
prefer-to-be	Suggest using toBe() for primitive literals	🎨		🔧
prefer-to-contain	Suggest using toContain()	🎨		🔧
prefer-to-have-length	Suggest using toHaveLength()	🎨		🔧
prefer-todo	Suggest using test.todo			🔧
require-hook	Require setup and teardown code to be within a hook
require-to-throw-message	Require a message for toThrow()
require-top-level-describe	Require test cases and hooks to be inside a describe block
valid-describe-callback	Enforce valid describe() callback	✅
valid-expect	Enforce valid expect() usage	✅
valid-expect-in-promise	Require promises that have expectations in their chain to be valid	✅
valid-title	Enforce valid titles	✅		🔧

Requires Type Checking

Name	Description	💼	⚠️	🔧	💡
unbound-method	Enforce unbound methods are called with their expected scope

In order to use the rules powered by TypeScript type-checking, you must be using @typescript-eslint/parser & adjust your eslint config as outlined here.

Note that unlike the type-checking rules in @typescript-eslint/eslint-plugin, the rules here will fallback to doing nothing if type information is not available, meaning it's safe to include them in shared configs that could be used on JavaScript and TypeScript projects.

Also note that unbound-method depends on @typescript-eslint/eslint-plugin, as it extends the original unbound-method rule from that plugin.

Credit

• eslint-plugin-mocha
• eslint-plugin-jasmine

Related Projects

eslint-plugin-jest-extended

This is a sister plugin to eslint-plugin-jest that provides support for the matchers provided by jest-extended.

https://github.com/jest-community/eslint-plugin-jest-extended

eslint-plugin-jest-formatting

This project aims to provide formatting rules (auto-fixable where possible) to ensure consistency and readability in jest test suites.

https://github.com/dangreenisrael/eslint-plugin-jest-formatting

eslint-plugin-istanbul

A set of rules to enforce good practices for Istanbul, one of the code coverage tools used by Jest.

https://github.com/istanbuljs/eslint-plugin-istanbul

Changelog

28.0.0-next.2 (2024-03-21)

Features

• drop support for Node v14 (#1527) (df5e580)

BREAKING CHANGES

• dropped support for Node v14