jest-expect-message

🃏🗯

Add custom message to Jest expects

---

Problem

In many testing libraries it is possible to supply a custom message for a given expectation, this is currently not possible in Jest.

For example:

test('returns 2 when adding 1 and 1', () => {
  expect(1 + 1, 'Woah this should be 2!').toBe(3);
});

This will throw the following error in Jest:

Expect takes at most one argument.

Solution

jest-expect-message allows you to call expect with a second argument of a String message.

For example the same test as above:

test('returns 2 when adding 1 and 1', () => {
  expect(1 + 1, 'Woah this should be 2!').toBe(3);
});

With jest-expect-message this will fail with your custom error message:

  ● returns 2 when adding 1 and 1

    Custom message:
      Woah this should be 2!

    expect(received).toBe(expected) // Object.is equality

    Expected: 3
    Received: 2

Installation

With npm:

npm install --save-dev jest-expect-message

With yarn:

yarn add -D jest-expect-message

Setup

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

Jest v24+

"jest": {
  "setupFilesAfterEnv": ["jest-expect-message"]
}

Jest v23-

"jest": {
  "setupTestFrameworkScriptFile": "jest-expect-message"
}

If you have a custom setup file and want to use this library then add the following to your setup file.

import 'jest-expect-message';

Configure Typescript

Add the following entry to your tsconfig to enable Typescript support.

  "files": ["node_modules/jest-expect-message/types/index.d.ts"],

Example

Custom message example with typescript

Configure ESlint

"rules": {
  "jest/valid-expect": [
    "error",
    {
      "maxArgs": 2
    }
  ]
}

Usage

• expect(actual, message, options?)
  • actual: The value you would normally pass into an expect to assert against with a given matcher.
  • message: String, the custom message you want to be printed should the expect fail.
  • options: An optional object that controls what is shown as part of the custom message.
    • showPrefix: boolean: If false will not show the Custom message: prefix. Default: true
    • showMatcherMessage: boolean: If false will not show the matchers original error message. Default: true
    • showStack: boolean: If false will not show the matchers stack trace. Default: true

test('returns 2 when adding 1 and 1', () => {
  expect(1 + 1, 'Woah this should be 2!').toBe(3);
});
// ↓ ↓ ↓ ↓ ↓ ↓
/*
  ● returns 2 when adding 1 and 1

    Custom message:
      Woah this should be 2!

    expect(received).toBe(expected) // Object.is equality

    Expected: 3
    Received: 2

  1 |   test('returns 2 when adding 1 and 1', () => {
> 2 |     expect(1 + 1, 'Woah this should be 2!').toBe(3);
    |                                             ^
  3 |   });
*/

showPrefix: false

test('returns 2 when adding 1 and 1', () => {
  expect(1 + 1, 'Woah this should be 2!', { showPrefix: false }).toBe(3);
});
// ↓ ↓ ↓ ↓ ↓ ↓
/*
  ● returns 2 when adding 1 and 1

    Woah this should be 2!

    expect(received).toBe(expected) // Object.is equality

    Expected: 3
    Received: 2

  1 |   test('returns 2 when adding 1 and 1', () => {
> 2 |     expect(1 + 1, 'Woah this should be 2!', { showPrefix: false }).toBe(3);
    |                                                                    ^
  3 |   });
*/

showMatcherMessage: false

test('returns 2 when adding 1 and 1', () => {
  expect(1 + 1, 'Woah this should be 2!', { showMatcherMessage: false }).toBe(3);
});
// ↓ ↓ ↓ ↓ ↓ ↓
/*
  ● returns 2 when adding 1 and 1

    Custom message:
      Woah this should be 2!

  1 |   test('returns 2 when adding 1 and 1', () => {
> 2 |     expect(1 + 1, 'Woah this should be 2!', { showMatcherMessage: false }).toBe(3);
    |                                                                            ^
  3 |   });
*/

showStack: false

test('returns 2 when adding 1 and 1', () => {
  expect(1 + 1, 'Woah this should be 2!', { showStack: false }).toBe(3);
});
// ↓ ↓ ↓ ↓ ↓ ↓
/*
  ● returns 2 when adding 1 and 1

    Custom message:
      Woah this should be 2!

    expect(received).toBe(expected) // Object.is equality

    Expected: 3
    Received: 2
*/

LICENSE

MIT