Implement a delay in ES6 Promise.

Promise.delay.js

Promise.prototype.delay = function(duration) {
  return this.then(function(value) {
    return new Promise(function(resolve) {
      setTimeout(function() {
        resolve(value)
      }, duration)
    })
  }, function(reason) {
    return new Promise(function(resolve, reject) {
      setTimeout(function() {
        reject(reason)
      }, duration)
    })
  })
}