Message passing in Chrome extensions

backgroundscript.js

chrome.extension.onMessage.addListener(
	/*
		request		= payload
		sender		= information about the payload
		sendResponse	= function to send response to. Explanation above (1).
	*/
	function(request, sender, sendResponse){
		var tabId = sender.tab.id,		// Which tab is the query from?
            	    isValid;
		
		/*
			Check the request is valid here
		*/
		
		if (!isValid) {
			sendMessage(tabId, 'Error');	// Send an error to the right tab
			return;				// Stop execution
		}
		
		getXHR('[atlasUrl]' + request, function(error, results){
			if (error) {
				sendMessage(tabId, 'Error');
			} else {
				sendMessage(tabId, results);
			}
		});
	}
);

function sendMessage(tab, data) {
	if (tab && data) {
		chrome.tabs.sendMessage(tab, data);
	}
}

contentscript.js

chrome.extension.onMessage.addListener(function(results) {	// Listen for results
	// Append results to the DOM
});

function sendMessage(data) {
	if (data) {
        	chrome.extension.sendMessage(data, false);	// false here can be replaced with
	}							// a function. Explanation below (1).
}