chat-mo.js

const DOMObserver = {
	parsers: [],
	watchers: [],
	queue: [],
	observer: new MutationObserver( records => {
		for( const record of records ) {
			DOMObserver.force(record);
		}
	}).observe(document.body, {
		subtree: true,
		childList: true
	}),
	addParser (parser, selector) {
		if( selector ) DOMObserver.queue.push(selector);
		DOMObserver.parsers.push(parser);
	},
	addWatcher (watcher) {
		DOMObserver.watchers.push(watcher);
	},
	drain() {
		for( const selector of DOMObserver.queue ) {
			DOMObserver.force({addedNodes: document.querySelectorAll(selector)});
		}
		DOMObserver.queue = [];
	},
	force({addedNodes, removedNodes = []}) {
		for( const addedNode of Array.from(addedNodes) ) {
			if( !addedNode.classList ) return;
			DOMObserver.parsers.forEach( parser => parser(addedNode));
		}
		for( const removedNode of Array.from(removedNodes) ) {
			DOMObserver.watchers.forEach(watcher => watcher(removedNode));
		}
	}
};

// ---------------------

function genericParser(node) {
	// node that was added, test for your elements here 
	if( node.classList && node.classList.contains('message') ) {
		// do shit
	}
}

DOMObserver.addParser(genericParser, '.user-container .message') // you need to pass a selector that will match the elements as well
// add as manyt parsers as you like. 

DOMObserver.drain(); // kick it off