task.js

function Task(fork) {
  return {
    fork,
    map: (mapRejector, mapResolver) => Task((reject, resolve) => {       
      fork(function() {
        if (mapRejector) { 
          reject(mapRejector.apply(null, arguments))
        } else {
          reject.apply(null, arguments)
        }
      }, function() {
        if (mapResolver) {
          resolve(mapResolver.apply(null, arguments))
        } else {
          resolve.apply(null, arguments)  
        }
      })
    })
  }
}

function mayEitherPassOrFail(reject, resolve) {
  let resolveData = '{"foo":"1","bar":"2","baz":"3"}',
      rejectData = '{"error":"Random reason"}'

  let cancelToken = setTimeout(() => {
    if (Math.random() < 0.5) {
      resolve(resolveData)
    } else {
      reject(rejectData)
    }
  }, 500);
  
  return () => clearTimeout(cancelToken)
}

var randomTask = Task(mayEitherPassOrFail)
var cancelFunction = randomTask
  .map(string => JSON.parse(string), string => JSON.parse(string))
  .map(null, obj =>  Object.assign({}, obj, { newKey: 'newValue' }))
  .fork(rejection => console.log(`The task was rejected with reason: ${rejection.error}`), 
    resolution => console.log(`Task completed with data keys: ${Object.keys(resolution)}`)
  )

//cancelFunction();