eslint-plugin-jest

ESLint plugin for Jest

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Quick Overview

ESLint-plugin-jest is a plugin for ESLint that provides linting rules for Jest, a popular JavaScript testing framework. It helps developers maintain consistent and error-free test code by enforcing best practices and catching common mistakes in Jest tests.

Pros

Improves code quality and consistency in Jest tests
Catches common errors and anti-patterns early in the development process
Customizable rules to fit specific project needs
Regularly updated to support the latest Jest features

Cons

May require initial setup and configuration time
Some rules might be too strict for certain coding styles or preferences
Can potentially increase build times due to additional linting checks
Occasional false positives that may need manual overrides

Code Examples

Configuring ESLint to use the Jest plugin:

{
  "extends": ["plugin:jest/recommended"],
  "plugins": ["jest"],
  "env": {
    "jest/globals": true
  }
}

Using the expect-expect rule to ensure expectations are made in tests:

// This test will pass the linting rule
test('example test', () => {
  expect(2 + 2).toBe(4);
});

// This test will fail the linting rule
test('example test without expect', () => {
  const result = 2 + 2;
  // Missing expect statement
});

Applying the valid-title rule to enforce proper test titles:

// This will pass the linting rule
describe('My Component', () => {
  it('should render correctly', () => {
    // Test code
  });
});

// This will fail the linting rule
describe('my component', () => {
  it('renders correctly', () => {
    // Test code
  });
});

Getting Started

To start using eslint-plugin-jest in your project:

Install the plugin:

npm install --save-dev eslint eslint-plugin-jest

Add the plugin to your ESLint configuration file (e.g., .eslintrc.js):

module.exports = {
  plugins: ['jest'],
  extends: ['plugin:jest/recommended'],
  env: {
    'jest/globals': true
  }
};

Run ESLint on your test files:
```
npx eslint path/to/your/test/files
```

You can customize the rules by adding them to the rules section of your ESLint configuration file.

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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

If you're using flat configuration:

With flat configuration, just import the plugin and away you go:

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

module.exports = [
  {
    // update this to match your test files
    files: ['**/*.spec.js', '**/*.test.js'],
    plugins: { jest: pluginJest },
    languageOptions: {
      globals: pluginJest.environments.globals.globals,
    },
    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',
    },
  },
];

With legacy configuration, add jest to the plugins section of your .eslintrc configuration file. You can omit the eslint-plugin- prefix:

{
  "plugins": ["jest"],
  "env": {
    "jest/globals": true
  },
  "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"
  }
}

[!NOTE]

You only need to explicitly include our globals if you're not using one of our shared configs

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"]
      }
    }
  }
}

Aliased `@jest/globals`

You can tell this plugin to treat a different package as the source of Jest globals using the globalPackage setting:

{
  "settings": {
    "jest": {
      "globalPackage": "bun:test"
    }
  }
}

[!WARNING]

While this can be used to apply rules when using alternative testing libraries and frameworks like bun, vitest and node, there's no guarantee the semantics this plugin assumes will hold outside of Jest

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.

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.

Rules

ð¼ Configurations enabled in.
â ï¸ Configurations set to warn in.
â Set in the recommended configuration.
ð¨ Set in the style configuration.
ð§ Automatically fixable by the --fix CLI option.
ð¡ Manually fixable by editor suggestions.

NameÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â	Description	ð¼	â ï¸	ð§	ð¡
consistent-test-it	Enforce `test` and `it` usage conventions			ð§
expect-expect	Enforce assertion to be made in a test body		â
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	â	ð¨	ð§
no-commented-out-tests	Disallow commented out tests		â
no-conditional-expect	Disallow calling `expect` conditionally	â
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	â		ð§
no-disabled-tests	Disallow disabled tests		â
no-done-callback	Disallow using a callback in asynchronous tests and hooks	â			ð¡
no-duplicate-hooks	Disallow duplicate setup and teardown hooks
no-export	Disallow using `exports` in files containing tests	â
no-focused-tests	Disallow focused tests	â			ð¡
no-hooks	Disallow setup and teardown hooks
no-identical-title	Disallow identical titles	â
no-interpolation-in-snapshots	Disallow string interpolation inside snapshots	â
no-jasmine-globals	Disallow Jasmine globals	â		ð§
no-large-snapshots	Disallow large snapshots
no-mocks-import	Disallow manually importing from `__mocks__`	â
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	â
no-test-prefixes	Require using `.only` and `.skip` over `f` and `x`	â		ð§
no-test-return-statement	Disallow explicitly returning from tests
no-untyped-mock-factory	Disallow using `jest.mock()` factories without an explicit type parameter			ð§
padding-around-after-all-blocks	Enforce padding around `afterAll` blocks			ð§
padding-around-after-each-blocks	Enforce padding around `afterEach` blocks			ð§
padding-around-all	Enforce padding around Jest functions			ð§
padding-around-before-all-blocks	Enforce padding around `beforeAll` blocks			ð§
padding-around-before-each-blocks	Enforce padding around `beforeEach` blocks			ð§
padding-around-describe-blocks	Enforce padding around `describe` blocks			ð§
padding-around-expect-groups	Enforce padding around `expect` groups			ð§
padding-around-test-blocks	Enforce padding around `test` and `it` blocks			ð§
prefer-called-with	Suggest using `toBeCalledWith()` or `toHaveBeenCalledWith()`
prefer-comparison-matcher	Suggest using the built-in comparison matchers			ð§
prefer-each	Prefer using `.each` rather than manual loops
prefer-equality-matcher	Suggest using the built-in equality matchers				ð¡
prefer-expect-assertions	Suggest using `expect.assertions()` OR `expect.hasAssertions()`				ð¡
prefer-expect-resolves	Prefer `await expect(...).resolves` over `expect(await ...)` syntax			ð§
prefer-hooks-in-order	Prefer having hooks in a consistent order
prefer-hooks-on-top	Suggest having hooks before any test cases
prefer-importing-jest-globals	Prefer importing Jest globals			ð§
prefer-jest-mocked	Prefer `jest.mocked()` over `fn as jest.Mock`			ð§
prefer-lowercase-title	Enforce lowercase test names			ð§
prefer-mock-promise-shorthand	Prefer mock resolved/rejected shorthands for promises			ð§
prefer-snapshot-hint	Prefer including a hint with external snapshots
prefer-spy-on	Suggest using `jest.spyOn()`			ð§
prefer-strict-equal	Suggest using `toStrictEqual()`				ð¡
prefer-to-be	Suggest using `toBe()` for primitive literals	ð¨		ð§
prefer-to-contain	Suggest using `toContain()`	ð¨		ð§
prefer-to-have-length	Suggest using `toHaveLength()`	ð¨		ð§
prefer-todo	Suggest using `test.todo`			ð§
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	â
valid-expect	Enforce valid `expect()` usage	â		ð§
valid-expect-in-promise	Require promises that have expectations in their chain to be valid	â
valid-title	Enforce valid titles	â		ð§

Requires Type Checking

NameÂ Â Â Â Â Â Â Â Â Â	Description	ð¼	â ï¸	ð§	ð¡
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

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