prom.js

function asyncIter(arr, fn, errCb) {
  var i = 0

  function runIt() {
    try {
      fn(arr[i], i, function() {
        i++
        if (i < arr.length) {
          setTimeout(runIt, 5)
        }
      })
    } catch (err) {
      return errCb(err)
    }
  }

  if (arr.length > 0) {
    setTimeout(runIt, 5)
  }
}


function Promise (fn) {
  this._thens = []
  this._catch = null
  this._completed = false

  try {
    fn(this._resolve.bind(this), this._reject.bind(this))
  } catch (err) {
    this._reject(err)
  }
}


Promise.prototype._resolve = function(result) {
  setTimeout(function() {
    this._result = result
    this._completed = true
    this._complete(result)
  }.bind(this), 5)

}

Promise.prototype._reject = function(err) {
  setTimeout(function() {
    this._complete(undefined, err)
  }.bind(this))
}

Promise.prototype.then = function(fn) {
  if (this._completed) {
    this._callThens()
  } else {
   this._thens.push(fn)
  }

  return this
}

Promise.prototype.catch = function(fn) {
 this._catch = fn
 return this
}

Promise.prototype._callThens = function() {
  var result = this._result

  asyncIter(this._thens, function(thenCb, i, next) {
   var prom = thenCb(result)

   if (prom instanceof Promise) {
     prom.then(function(res) {
       result = res
       next()
     })
     if (this._catch) {
       prom.catch(this._catch.bind(this))
     }
   } else {
     result = prom
     next()
   }
  }.bind(this), function(err) {
    this._reject(err)
  }.bind(this))
}

Promise.prototype._complete = function(result, error) {
  if (result) {
    try {
      this._callThens()
    } catch (err) {
      if (this._catch) {
        this._catch(err)
      } else {
        throw new Error('Uncaugt Promise rejection!')
      }
    }
  } else if (error && this._catch) {
    this._catch(error)
  }
}


var prom = new Promise(function(resolve, reject) {
  console.log('bem')
  resolve('ahoj')
})

prom.then(function(res) {
  console.log('heh')
  return new Promise(function(resolve) {
    resolve('cau')
  })
})
.then(function(result) {
  console.log('cau?', result)
  return result
})
//.then(function(res) {
//  throw new Error('erooor')
//})
.catch(function(err) {
  console.log('bubu', err)
})

console.log('bum')