gist5.js

function imageMatch(file) {
  return readImageAsPromise(file).then(image => {
    return of(image).map(itShouldProp('width', large(10), always('image width should large 10, now is ' + image.width)))
  })
}

of({ filename: 'c:/fakepath/file.txt'})
  .map(itShould(imageMatch, always('image not exists')))
  .validate(( success, reason) => {
   // async validation just fun
  })