I'm writing a pixel library to do an AJAX request. So, I'm not allowed to use any helper.

function ajax(options) {
  var xhr = new XMLHttpRequest();
  var opts = Object.assign({
    withCredentials: false,
    method: 'GET'
  }, options);

  xhr.withCredentials = options.withCredentials;
  xhr.open(opts.method, opts.url);
  xhr.setRequestHeader('Accept', 'text/plain');
  xhr.send(null);
  return {
    done: function(cb) {
      xhr.onreadystatechange = function onStateChange() {
        if (this.readyState === 4) {
          if (this.status >= 200 && this.status < 300) {
            cb(this.responseText);
          } else {
            cb('error');
          }
        }
      };
      xhr.ontimeout = function(e) {
        console.error(e);
        cb(false);
      };

      xhr.onerror = function(e) {
        console.error(e);
        cb(false);
      };
    }
  }
};

ajax({
  url: '/echo/json'
}).done(function(response) {
  console.log(response);
})

I also have jsfiddle in case anyone wants to test the code. https://jsfiddle.net/5ch7b4jo/2/

This library doesn't support POST as I want to keep the code as less as possible. It will only support GET.

I'm writing a pixel library to do an AJAX request so I'm not allowed to use any helper.

function ajax(options) {
  var xhr = new XMLHttpRequest();
  var opts = Object.assign({
    withCredentials: false,
    method: 'GET'
  }, options);

  xhr.withCredentials = options.withCredentials;
  xhr.open(opts.method, opts.url);
  xhr.setRequestHeader('Accept', 'text/plain');
  xhr.send(null);
  return {
    done: function(cb) {
      xhr.onreadystatechange = function onStateChange() {
        if (this.readyState === 4) {
          if (this.status >= 200 && this.status < 300) {
            cb(this.responseText);
          } else {
            cb('error');
          }
        }
      };
      xhr.ontimeout = function(e) {
        console.error(e);
        cb(false);
      };

      xhr.onerror = function(e) {
        console.error(e);
        cb(false);
      };
    }
  }
};

ajax({
  url: '/echo/json'
}).done(function(response) {
  console.log(response);
})

I also have my code in a jsfiddle in case anyone wants to test the code.

Note: This library doesn't support POST as I want to keep the code as less as possible. It will only support GET.