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 anexpect
to assert against with a given matcher.message
: String, the custom message you want to be printed should theexpect
fail.options
: An optional object that controls what is shown as part of the custom message.showPrefix: boolean
: Iffalse
will not show theCustom message:
prefix. Default:true
showMatcherMessage: boolean
: Iffalse
will not show the matchers original error message. Default:true
showStack: boolean
: Iffalse
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 | });
*/
false
showPrefix: 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 | });
*/
false
showMatcherMessage: 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 | });
*/
false
showStack: 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
*/