jest-chain

• 🍸 Less code duplication
• 🤗 Chain core and custom matchers together
• 👾 Expressive assertions
• 🚨 Fail fast assertions

Problem

Often in Jest when you are writing tests you may want to perform multiple assertions on the same variable. Currently to achieve this you have to write an individual expect for each assertion.

For example:

it('add 1 and 1', () => {
  const actual = 1 + 1;
  expect(actual).toBe(2);
  expect(actual).toBeGreaterThan(1);
  expect(actual).toBeLessThan(3);
});

With jest-chain this can instead be written by chaining the matchers together:

it('add 1 and 1', () => {
  expect(1 + 1)
    .toBe(2)
    .toBeGreaterThan(1)
    .toBeLessThan(3);
});

Installation

With npm:

npm install --save-dev jest-chain

With yarn:

yarn add -D jest-chain

Setup

Add jest-chain to your Jest setupFilesAfterEnv configuration. See for help

Jest >v24

"jest": {
  "setupFilesAfterEnv": ["jest-chain"]
}

Jest <v23

"jest": {
  "setupTestFrameworkScriptFile": "jest-chain"
}

If you are already using another test framework, like jest-extended, then you should create a test setup file and require each of the frameworks you are using (including jest-chain 😉)

For example:

// ./testSetup.js
require('jest-chain');
require('any other test framework libraries you are using');

Then in your Jest config:

"jest": {
  "setupTestFrameworkScriptFile": "./testSetup.js"
}

Typescript

If your editor does not recognise the chained jest matchers, add a global.d.ts file to your project with:

import 'jest-chain';

Note: if you are using any other custom matcher libraries then make sure that the jest-chain type import is at the bottom so that the types can chain core matchers with your customer matcher library.

Usage

Use Jest's expect function the same way you would normally but with the ability to chain any matcher to another, including nested matchers such as: .not, .resolves and .rejects.

jest-chain supports custom Jest matchers, like jest-extended, in the usual way with expect.extend(matcher). Each of these custom matchers are also chainable.

Some examples:

expect([1, 2, 3])
  .toHaveLength(3)
  .toEqual([1, 2, 3]);

// with jest-extended
expect([1, 2, 3])
  .toBeArray()
  .toBeArrayOfSize(3)
  .toEqual([1, 2, 3])
  .toIncludeAnyMembers([1, 2]);

expect(100)
  .toBePositive()
  .toBeGreaterThan(99)
  .toBeLessThan(101)
  .toBeNumber()
  .not.toBeNaN()
  .toBe(100);

expect('hello world')
  .toBeString()
  .toEqualCaseInsensitive('HELLO WORLD')
  .toStartWith('hello')
  .toEndWith('world')
  .not.toInclude('!')
  .toBe('hello world');

Matcher failures will fail fast from left to right, they have no impact on each other. 🎉

Note: jest-chain does not currently support asymmetric matcher chaining, if you want this please send a PR 😊

LICENSE

MIT