canned.js

const KEYWORD_MAP = {};
/*
	KEYWORD_MAP[<user_id>][<term>] returns the reply
*/
KEYWORD_MAP[774078] = {
	'jesus': 'http://i.imgur.com/V1kcfEU.jpg'
};
KEYWORD_MAP[617762] = {
	'BAM!' : 'SPACE GUN!'
};

function keywordParser(node) {
	if( !node.classList.contains('message') || node.classList.contains('pending') ) {
		return;
	}
	const sig = node.parentNode.parentNode.querySelector('a.signature');
	if( !sig ) {
		return;
	}
	const [,,userId] = sig.getAttribute('href').split('/');
	if( !(Number(userId) in KEYWORD_MAP) ) {
		return;
	}
	const text = node.textContent;
	
	// right now we only trigger on the first found word. 
	const matchedKey = Object.keys(KEYWORD_MAP[userId]).find(key => text.includes(key));
	if( !matchedKey ) {
		return;
	}
	const reply = KEYWORD_MAP[userId][matchedKey];

	const [,messageId] = node.id.split('-');

	sendMessage(messageId, reply);
}

function sendMessage(messageId, reply) {
	const [roomid] = /\d+/.exec(location);

	fetch(`https://chat.stackoverflow.com/chats/${roomid}/messages/new`, {
		credentials: 'same-origin', 
		method: 'POST', 
		headers: { 'Content-Type': 'application/x-www-form-urlencoded; charset=utf-8' },
		body: `fkey=${fkey().fkey}&text=:${messageId} ${reply}`
	}).catch( error => {
		console.log(error);
		setTimeout(() => {
			sendMessage(messageId, reply);
		}, 1000);
	});
}

DOMObserver.addParser(keywordParser, '.user-container .message');