Assert Rejects

Assert that a promise eventually rejects

Installation

npm install --save assert-rejects

Usage

const 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')
  })
})

API

assertRejects(promise[, error][, message])

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')