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JavaScript AJAX Basics (retiring) Programming AJAX AJAX Callbacks

Dustin Henry
Dustin Henry
7,392 Points
Posted by Dustin Henry
Dustin Henry
7,392 Points

Why do we do xhr.onreadystatechange = function instead of xhr.onreadystatechange(function)?

For instance, if I was adding an Event Handler to a DOM level object, I would do object.addEventListener('click', function), but with the onreadystatechange event handler, an equals sign is used instead. Why is that? Thank you for the help!

1 Answer

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

The "addEventListener" method establishes an event handler for an object, but "onreadystatechange" is an event handler.

It might make more sense if we compare two ways of setting up "myClickHandler" on some object:

myButton.addEventListener('click', myClickHandler);  // this is one way
myButton.onclick = myClickHandler;                   // this is another

Both of those do (almost) the same thing. The advantage of "addEventListener" is that you can attach more than one handler that way.