Use this in a .slate.js configuration to cycle through a list of applications.

cycler.js

// Usage:
//
// slate.bind("1:cmd;alt", Cycler(["Safari", "Google Chrome", "Firefox"]));
//

var Cycler = (function() {
	function Cycler(appNames, cycleWindows) {
		this.appNames = appNames;
		this.cycleWindows = cycleWindows;
		this.lastIndex = -1;

		var that = this;

		this.callback = function(win) {
			var apps = getApps(that.appNames);

			if (apps.length === 0) { return; }

			if (anyHasFocus(apps)) {
				that.lastIndex = cycle(that.lastIndex, apps);
			} else {
				if (that.lastIndex === -1) { that.lastIndex = 0; }
				var toFocus = apps[that.lastIndex];
				if (toFocus !== undefined) {
					focusApp(toFocus);
				}
			}
		};
	}

	function getApps(names) {
		var apps = [];
		slate.eachApp(function(app) {
			if (_.contains(names, app.name())) {
				apps.push(app);
			}
		});
		return _.sortBy(apps, function(app) { return app.pid(); });
	}

	function anyHasFocus(apps) {
		var focusedApp = slate.app();
		return _.some(apps, function(app) {
			return focusedApp.pid() === app.pid();
		});
	}

	function cycle(lastIndex, apps) {
		var newIndex = (lastIndex + 1) % apps.length;
		var appToFocus = apps[newIndex];
		slate.log("appToFocus: " + appToFocus.name());

		// cycle again if the current app in focus is already the app which was going to be focused
		if (slate.app().pid() === appToFocus.pid() && apps.length > 1) {
			slate.log("cycle again");
			return cycle(newIndex, apps);
		}

		focusApp(appToFocus);

		return newIndex;
	}

	function focusApp(app) {
		var main = app.mainWindow();
		if (main !== null) {
			main.focus();
		}
	}

	return function(appNames, cycleWindows) {
		return new Cycler(appNames, cycleWindows).callback;
	};
})();