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JavaScript Asynchronous Programming with JavaScript Asynchronous JavaScript with Callbacks Async Programming and Callback Functions


OK, this may sound silly... but how does the browser know I am requesting an AJAX for the astrosUrl? or can the function below be re-used for other requests?

function getJSON(url) {
  const xhr = new XMLHttpRequest();
  xhr.open('GET', url);
  xhr.onload = () => {
    if(xhr.status === 200) {
      let data = JSON.parse(xhr.responseText);

I mean, I assume, here is when we call that particular URL, isn't it? but can the above be re-used for the others URL?

btn.addEventListener('click', () => getJSON(astrosUrl));

Also, why aren't we using:

(xhr.status === 200 && xhr.readyState === 4)


1 Answer

Steven Parker
Steven Parker
210,423 Points

The "getJSON" function knows which URL to get data from because the URL is passed to it as an argument when it is called.

This function isn't re-used because it only logs the data to the console, and the other task displays it on the page.

And it's not necessary to test "readyState" since the "onload" only runs when readyState is 4.