Javascript snippet to asynchronously detect when DOM is loaded

wait-for-dom-content-loaded.js

class SampleApp() {
    
    constructor () {
        this.pageLoadingPromise = null;
    }
    
    waitForDomContentLoaded() {
        if (!this.pageLoadingPromise) {
            // this is the first time; prepare the promise
            this.pageLoadingPromise = new Promise(resolve => {
                // make sure that page is not already loaded (otherwise we'd listen forever)
                if (document.readyState === 'loading') {
                    // document is still loading, so register listener
                    document.addEventListener("DOMContentLoaded", resolve);
                } else {
                    // we are past loading, at least "interactive" level, and that's just what we need
                    resolve();
                }
            });
        }
        // subsequent calls are just going to return the original promise, be it fulfilled or not
        return this.pageLoadingPromise;
    }
}