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

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.

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

Recommended

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

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

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

See ESLint documentation for more information about extending configuration files.

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"]
}

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

Rule	Description	Configurations	Fixable
consistent-test-it	Have control over test and it usages
expect-expect	Enforce assertion to be made in a test body
lowercase-name	Enforce lowercase test names
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	Prevent calling expect conditionally
no-deprecated-functions	Disallow use of deprecated functions
no-disabled-tests	Disallow disabled tests
no-done-callback	Avoid 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-if	Disallow conditional logic
no-interpolation-in-snapshots	Disallow string interpolation inside snapshots
no-jasmine-globals	Disallow Jasmine globals
no-jest-import	Disallow importing Jest
no-large-snapshots	disallow large snapshots
no-mocks-import	Disallow manually importing from __mocks__
no-restricted-matchers	Disallow specific matchers & modifiers
no-standalone-expect	Disallow using expect outside of it or test blocks
no-test-prefixes	Use .only and .skip over f and x
no-test-return-statement	Disallow explicitly returning from tests
prefer-called-with	Suggest using toBeCalledWith() or toHaveBeenCalledWith()
prefer-expect-assertions	Suggest using expect.assertions() OR expect.hasAssertions()
prefer-expect-resolves	Prefer await expect(...).resolves over expect(await ...) syntax
prefer-hooks-on-top	Suggest having hooks before any test cases
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	Enforce valid describe() callback
valid-expect	Enforce valid expect() usage
valid-expect-in-promise	Ensure promises that have expectations in their chain are valid
valid-title	Enforce valid titles

TypeScript Rules

In addition to the above rules, this plugin also includes a few advanced rules that are powered by type-checking information provided by TypeScript.

In order to use these rules, 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 its 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.

Rule	Description	Configurations	Fixable
unbound-method	Enforces unbound methods are called with their expected scope

Credit

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

Related Projects

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

24.7.0 (2021-10-10)

Features

• create require-hook rule (#929) (6204b31)
• deprecate prefer-to-be-null rule (4db9161)
• deprecate prefer-to-be-undefined rule (fa08f09)