Assert that a promise eventually rejects
npm install --save assert-rejectsconst assertRejects = require('assert-rejects')
describe('Something', () => {
it('rejects', () => {
const promise = doSomethingThatShouldReject()
return assertRejects(promise)
})
it('rejects with specific code', () => {
const promise = readFileThatDoesntExists()
return assertRejects(promise, err => err.code === 'ENOENT')
})
})Expects the promise to reject. Returns a new promise that will resolve once the provided promise is resolved.
If specified, error can be a constructor, RegExp, or validation function.
If specified, message will be the message provided by the AssertionError if the promise fails to reject.
Validate instanceof using constructor:
assertRejects(
Promise.reject(new Error('Wrong value')),
Error
)Validate error message using RegExp:
assertRejects(
Promise.reject(new Error('Wrong value')),
/value/
)Custom error validation:
assertRejects(
Promise.reject(new Error('Wrong value')),
(err) => ((err instanceof Error) && /value/.test(err)),
'unexpected rejection'
)Note that error can not be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes:
// THIS IS A MISTAKE! DO NOT DO THIS!
assertRejects(myPromise, 'missing foo', 'did not reject with expected message')
// Do this instead.
assertRejects(myPromise, /missing foo/, 'did not reject with expected message')