SOURCE

function P(callback) {
  this.status = 'pending';
  this.value = '';
  var that = this;
  function resolve(params) {
    that.status = 'resolved'that.value = params
  }
  function reject(params) {
    that.status = 'rejected'that.value = params
  }
  callback(resolve, reject)
}
P.prototype.then = function(fn) {
  if (this.status === 'resolved') {
    var value = fn(this.value) if (value instanceof P) {
      this.status = value.status;
      this.value = value.value
    } else {
      this.status = 'resolved'this.value = value
    }
  }
  return this
}
P.prototype.
catch = function(fn) {
  if (this.status === 'rejected') {
    var value = fn(this.value) if (value instanceof P) {

      this.status = value.status;
      this.value = value.value
    } else {
      this.status = 'resolved'this.value = value
    }
  }
  return this
}