findFeatureId.js

var featureCollection;
var oneFeature;
var site;
var guid

function findFeatureId() {
	var clientContext = new SP.ClientContext();
	site = clientContext.get_web();
	clientContext.load(site);
	featureCollection = site.get_features();
	clientContext.load(featureCollection);
	clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
}
function onQuerySucceeded() {
	var listEnumerator = featureCollection.getEnumerator();
	var featureInfo = '';
	while (listEnumerator.moveNext()) {
		oneFeature = listEnumerator.get_current();
		featureInfo += 'Feature ID: ' + oneFeature.get_definitionId() + '\n';
		guid = oneFeature.get_definitionId();
	}
	alert(featureInfo);
}
function onQueryFailed(sender, args) {
	alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}