index.js

// Best place to find information on XHR features is:
// https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest

var reqfields = [
  'responseType', 'withCredentials', 'timeout', 'onprogress'
]

// Simple and small ajax function
// Takes a parameters object and a callback function
// Parameters:
//  - url: string, required
//  - headers: object of `{header_name: header_value, ...}`
//  - body:
//      + string (sets content type to 'application/x-www-form-urlencoded' if not set in headers)
//      + FormData (doesn't set content type so that browser will set as appropriate)
//  - method: 'GET', 'POST', etc. Defaults to 'GET' or 'POST' based on body
//  - cors: If your using cross-origin, you will need this true for IE8-9
//
// The following parameters are passed onto the xhr object.
// IMPORTANT NOTE: The caller is responsible for compatibility checking.
//  - responseType: string, various compatability, see xhr docs for enum options
//  - withCredentials: boolean, IE10+, CORS only
//  - timeout: long, ms timeout, IE8+
//  - onprogress: callback, IE10+
//
// Callback function prototype:
//  - statusCode from request
//    + Possibly null or 0 (i.e. falsy) if an error occurs
//  - response
//    + if responseType set and supported by browser, this is an object of some type (see docs)
//    + otherwise if request completed, this is the string text of the response
//    + if request is aborted, this is "Abort"
//    + if request times out, this is "Timeout"
//    + if request errors before completing (probably a CORS issue), this is "Error"
//  - request object
//
// Returns the request object. So you can call .abort() or other methods
//
// DEPRECATIONS:
//  - Passing a string instead of the params object has been removed!
//
exports.ajax = function (params, callback) {
  // Any variable used more than once is var'd here because
  // minification will munge the variables whereas it can't munge
  // the object access.
  var headers = params.headers || {}
    , body = params.body
    , method = params.method || (body ? 'POST' : 'GET')
    , called = false

  var req = getRequest(params.cors)

  function cb(statusCode, responseText) {
    return function () {
      if (!called) {
        callback(req.status === undefined ? statusCode : req.status,
                 req.status === 0 ? "Error" : (req.response || req.responseText || responseText),
                 req)
        called = true
      }
    }
  }

  req.open(method, params.url, true)

  var success = req.onload = cb(200)
  req.onreadystatechange = function () {
    if (req.readyState === 4) success()
  }
  req.onerror = cb(null, 'Error')
  req.ontimeout = cb(null, 'Timeout')
  req.onabort = cb(null, 'Abort')

  if (body) {
    setDefault(headers, 'X-Requested-With', 'XMLHttpRequest')

    if (!global.FormData || !(body instanceof global.FormData)) {
      setDefault(headers, 'Content-Type', 'application/x-www-form-urlencoded')
    }
  }

  for (var i = 0, len = reqfields.length, field; i < len; i++) {
    field = reqfields[i]
    if (params[field] !== undefined)
      req[field] = params[field]
  }

  for (var field in headers)
    req.setRequestHeader(field, headers[field])

  req.send(body)

  return req
}

function getRequest(cors) {
  // XDomainRequest is only way to do CORS in IE 8 and 9
  // But XDomainRequest isn't standards-compatible
  // Notably, it doesn't allow cookies to be sent or set by servers
  // IE 10+ is standards-compatible in its XMLHttpRequest
  // but IE 10 can still have an XDomainRequest object, so we don't want to use it
  if (cors && global.XDomainRequest && !/MSIE 1/.test(navigator.userAgent))
    return new XDomainRequest
  if (global.XMLHttpRequest)
    return new XMLHttpRequest
}

function setDefault(obj, key, value) {
  obj[key] = obj[key] || value
}