promise-switch.js

function switcher() {
  var on = true;
  var self = {};
  self.wire = function(val) {
    if (on) return val;
    throw new Error('Cancelled');
  };
  self.off = function() { on = false; }
  self.on = function() { on = true; }
  return self;
}

// Example usage:

function pageUnchanged() {
  var routeSwitch = switcher();
  router.onPageChange(routeSwitch.off);
  return routeSwitch.wire;
}

fetch(url).then(pageUnchanged()).then(updateUI);