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+
Add this to package.json
"jest": {
"setupFilesAfterEnv": ["jest-expect-message"]
}
Or if you encountered with this error "We detected setupFilesAfterEnv in your package.json ..."
, undo setupFilesAfterEnv
and create a file src/setupTests.js
containing a requirement statement:
// src/setupTests.js
require("jest-expect-message")
Jest v23-
"jest": {
"setupTestFrameworkScriptFile": "jest-expect-message"
}
Usage
-
expect(actual, message, wrapper)
-
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. -
wrapper (optional)
: Function, if you want to wrap the message with a custom text instead of default"Custom message:..."
.
-
test('returns 2 when adding 1 and 1', () => {
expect(1 + 1, 'Woah this should be 2!').toBe(3);
});
// with a custom wrapper
test('returns 2 when adding 1 and 1', () => {
const wrapper = msg => `| Custom Message << ${msg} >> |`
expect(1 + 1, 'Woah this should be 2!', wrapper).toBe(3);
});
Contributors
Matt Phillips |
Mohammad |
---|