when-failure.js

var when1 = require('when1');
var when2 = require('when2');

when1.Promise.onPotentiallyUnhandledRejectionHandled = function(r) {
  console.log('when1 Handled', r);
}

var rejectedPromise = when1.reject(new Error('Intentional failure'));
when2(rejectedPromise).done(undefined, function(error){
	console.log('Rejection properly handled:', error);
});

when-rejection-api.js

var when1 = require('when1');
var when2 = require('when2');

var rejections = [];
var timeout;
when1.Promise.onPotentiallyUnhandledRejection = function(r) {
    rejections.push(r);
    if(!timeout) timeout = setTimeout(report, 100);
}
when1.Promise.onPotentiallyUnhandledRejectionHandled = function(r) {
    rejections.splice(rejections.indexOf(r), 1);
}
function report() {
    timeout = void 0;
    rejections.forEach(function(r) {
        if(!r.handled) {
          console.error(r.value && r.value.stack || r.value);
        }
    });
    rejections = [];
}

var rejectedPromise = when1.reject(new Error('Intentional failure'));
when2(rejectedPromise).done(undefined, function(error){
	console.log('Rejection properly handled:', error);
});