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JavaScript AJAX Basics (retiring) Programming AJAX Parsing JSON Data

Posted by jonroby
jonroby
16,113 Points

Why doesn't the onreadystatechange event need a parentheses?

In the video onreadystatechange doesn't have a parentheses after. Why not?

2 Answers

Andrew Chalkley
STAFF
Andrew Chalkley
Treehouse Guest Teacher
Andrew Chalkley
Andrew Chalkley
Treehouse Guest Teacher

Hi there,

The onreadystatechange is a method that your provide on the xhr object. If you used parentheses you're firing it.

Here's an example with the onclick method on a button:

<html>
<body>
      <button>Greeting</button>
      <script>
            var button = document.querySelector("button");
            button.onclick = function() {
                  alert("Hello World");
            }
            button.onclick();
      </script>
</body>
</html>

This will automatically trigger the onclick method and show the alert.

What you're doing by setting the method to onreadystatechange is providing a method for the browser to call when it's ready. The difference between onclick example above is that we were triggering the event, the browser is triggering it in the AJAX example.

Regards
Andrew

Nicholas Mejia
Nicholas Mejia
23,800 Points
Nicholas Mejia
Nicholas Mejia
23,800 Points

onreadystatechange doesn't have parentheses because it doesn't take arguments and doesn't need to. It exists to contain a function that executes automatically when the readyState changes. If you need to pass an argument, you can do it in the parentheses that are part of the function contained in the onreadystatechange value.