Not pretty, not chainable, but more error tolerant promise using knockout observable

kopromise.js

;(function(ko) {
    ko.promise = function() {
    var self = this
    var promise = ko.observableArray([])

    promise.get = function(url) {
      var request = new XMLHttpRequest();
      request.promise = ko.observable({})
      request.open('GET', url, true)

      request.onload = function() {
        if (request.status >= 200 && request.status < 400) {
          request.promise({success: true, text: request.responseText})
        } else {
          request.promise({success: false, text: "server returned error"})
        }
      };

      request.onerror = function() {
          request.promise({success: false, text: "connection error"})
      };

      request.send()
      return request.promise
    }

    promise.all = function() {
      self.completed = []
      var narg = arguments.length
      for (var i = 0; i < narg; i++){
        arguments[i].subscribe(function(a) {
          self.completed.push(a)
          if(self.completed.length == narg) {
            promise(self.completed)
          }
        })
      }
    }
    return promise
  }
})(ko)

Not pretty, not chainable, but more error tolerant promise using knockout observable
use:

// multi
var p = new ko.promise()
p.all(p.get('a.json'), p.get('b.json'))
p.subscribe(function(data) {
  data.map(function(v) {
    console.log('all returned', v)
  })
})

// single
var c = p.get('c.json')
c.subscribe(function(data) {
  console.log('single get', data)
})