Question

The question is so like a zillion others here and on the web – How to check if DOM has loaded in Javascript? But here’s the catch:

• Without using a framework like jQuery etc;
• Without knowing if your script has been loaded via a statically placed <script> tag or via some other Javascript much later after the DOM has already loaded.

Can this be done more or less reliably and with cross-browser compatibility?

Added: Let me clarify: I’m writing a standalone .JS file which can be included in arbitrary webpages. I want to execute code AFTER the DOM has been loaded. But I don’t know HOW my script will be included. It could be by placing a <script> tag (in which case the traditional onload or DOM-readiness solutions will work); or it could be loaded via AJAX or some other means, much later after the DOM is already loaded (so the previously mentioned solutions will never fire).

Answer 1

The document.readyState property can be used to check if the document is ready. From MDN:

Values

The readyState of a document can be one of following:

• loading – The document is still loading.
• interactive – The document has finished loading and the document has been parsed but sub-resources such as images, stylesheets and frames are still loading.
• complete – The document and all sub-resources have finished loading. The state indicates that the load event is about to fire.

Code example:

if(document.readyState === "complete") {
    // Fully loaded!
}
else if(document.readyState === "interactive") {
    // DOM ready! Images, frames, and other subresources are still downloading.
}
else {
    // Loading still in progress.
    // To wait for it to complete, add "DOMContentLoaded" or "load" listeners.

    window.addEventListener("DOMContentLoaded", () => {
        // DOM ready! Images, frames, and other subresources are still downloading.
    });

    window.addEventListener("load", () => {
        // Fully loaded!
    });
}