Add custom message to Jest expects
npm install jest-expect-messageššÆ
Add custom message to Jest expects








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:
``js`
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:
`sh`
Expect takes at most one argument.
jest-expect-message allows you to call expect with a second argument of a String message.
For example the same test as above:
`js`
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:
`sh
ā 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
`
With npm:
`sh`
npm install --save-dev jest-expect-message
With yarn:
`sh`
yarn add -D jest-expect-message
Add jest-expect-message to your Jest setupFilesAfterEnv configuration.
See for help
`json`
"jest": {
"setupFilesAfterEnv": ["jest-expect-message"]
}
`json`
"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.
`js`
import 'jest-expect-message';
Add the following entry to your tsconfig to enable Typescript support.
`json`
"files": ["node_modules/jest-expect-message/types/index.d.ts"],
#### Example
Custom message example with typescript
`json`
"rules": {
"jest/valid-expect": [
"error",
{
"maxArgs": 2
}
]
}
- 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: trueshowMatcherMessage: boolean
- : If false will not show the matchers original error message. Default: trueshowStack: boolean
- : If false will not show the matchers stack trace. Default: true
`js
test('returns 2 when adding 1 and 1', () => {
expect(1 + 1, 'Woah this should be 2!').toBe(3);
});
// ā ā ā ā ā ā
/*
ā 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 | });
*/
`
`js
test('returns 2 when adding 1 and 1', () => {
expect(1 + 1, 'Woah this should be 2!', { showPrefix: false }).toBe(3);
});
// ā ā ā ā ā ā
/*
ā 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 | });
*/
`
`js
test('returns 2 when adding 1 and 1', () => {
expect(1 + 1, 'Woah this should be 2!', { showMatcherMessage: false }).toBe(3);
});
// ā ā ā ā ā ā
/*
ā 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 | });
*/
`
`js
test('returns 2 when adding 1 and 1', () => {
expect(1 + 1, 'Woah this should be 2!', { showStack: false }).toBe(3);
});
// ā ā ā ā ā ā
/*
ā 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
*/
``