Quick XHR API intro

XHR.js

const ajax = (method, url, success) => {
  // Create the new XHR
  const xhr = new XMLHttpRequest();

  // "Open" the request. I.e. configure its method and request URL
  xhr.open(method, url);
  
  // Set up a success handler. For XHR we need to respond to changes in the readyState. However,
  // the readyState can change for a number of reasons and we are only interested in the change
  // that represents completion of the request. So we check whether readyState is equal to
  // xhr.DONE, and if it is then we also check that the status was a success (200)
  xhr.onreadystatechange = () => {
    if (xhr.readyState === xhr.DONE && xhr.status === 200) {
      success(xhr.responseText);
    }
  };
  
  // Actually send the request
  xhr.send();

  return xhr;
};

// Example:
// ajax(
//   'GET',
//   'https://api.github.com/users/iansinnott',
//   (responseText) => console.log(responseText)
// );

NOTE: There is no error handling built into this function. It basically won't do anything if the server responds with a non-200 status code. This is a significant limitation, but this example is just to remind myself in a simple case how the xhr API works.