1-cycle.js

require('when/monitor/console');
var when = require('when');

// Creates a cycle p1 -> p2 -> p3 -> p4 -> p1
var p1 = when.promise(function(r1) {
	var p2 = when.promise(function(r2) {
		var p3 = when.promise(function(r3) {
			var p4 = when.promise(function(r4) {
				setTimeout(function() {
					r4(p1); // Cycle! resolve p4 with p1
				}, 100);
			});
			r3(p4);
		});
		r2(p3);
	});
	r1(p2);
});

2-output.txt

TypeError: Promise cycle
    at null._onTimeout (/Users/brian/Projects/cujojs/when/experiments/cycle.js:10:6)
[object Object]
    at /Users/brian/Projects/cujojs/when/experiments/cycle.js:8:18
    at /Users/brian/Projects/cujojs/when/experiments/cycle.js:7:17