documentattachEventonreadystatechange

Document.attachEvent onreadystatechange refers to a legacy Internet Explorer mechanism for binding a handler to the document’s onreadystatechange event. It was used in older IE versions (approximately IE 5–8) as a non-standard alternative to the standard addEventListener approach.

The onreadystatechange event fires when the document’s readyState changes. The common states are loading, interactive, and complete. In practice, developers often wait for readyState to become complete before accessing the DOM or performing dependent initialization. In old IE, you would attach a function with document.attachEvent("onreadystatechange", handler). Note that in these environments the event object is accessed as window.event, not as a parameter passed to the handler.

Compatibility and deprecation: attachEvent is non-standard and is not supported in modern browsers. It has different semantics from addEventListener, including differences in event object handling and propagation. For new code, use document.addEventListener("readystatechange", handler) or, more commonly, the DOMContentLoaded or window.onload events to run code after the document is ready.

Example (old IE):

document.attachEvent("onreadystatechange", function () {

if (document.readyState === "complete") {

// initialization code here

}

});

Today, developers should avoid document.attachEvent and onreadystatechange bindings in favor of standards-compliant patterns, ensuring cross-browser compatibility by feature-detecting support and using addEventListener or alternative ready-time hooks.