AJAX request helper (plain Javascript)

ajax-helper.js

var ajax = {

  newRequest: function(method, url, params, callback, context) {
    this.context = context;
    this.req = new XMLHttpRequest();
    this.req.open(method, url, true);

    if(params) {
      this.params = this.encodeParams(params);
    } else {
      this.params = '';
    }

    if(method === 'POST') {
      this.req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
    }

    var _this = this;

    this.req.onload = function() {
      if(this.status >= 200 && this.status < 400) {
        // AJAX succeed
        var data;
        if(this.responseText == 0) {
          data = { error: 'Invalid action.' };
        } else {
          data = JSON.parse(this.responseText);
        }
        callback.call(_this.context, data);
      } else {
        // Server error
        callback.call(_this.context, { error: 'Server error.' });
      }
    };

    this.req.onerror = function() {
      // Connection error
      callback.call(_this.context, { error: 'Connection error.' });
    };

    this.sendRequest();
  },

  encodeParams: function(params) {
    // Encode params object to URL params
    var encodedParams = Object.keys(params).map(function(k) {
      return encodeURIComponent(k) + '=' + encodeURIComponent(params[k]);
    }).join('&');

    return encodedParams;
  },

  sendRequest: function() {
    // Send the request
    this.req.send(this.params);
  }

};