Solution to http://bitsquest.bitbucket.org/index.html#4 , even though it's not necessary to use the variable (you can just go up and right till you hit the right wall, then down)

puzzle4.js

var beenTop = false;
 
this.on("start", function() {
    this.thrusters.left(true);
});

this.on('sensor:top', function(contact) {
  this.thrusters.bottom(false);
  beenTop = true;
});

this.on('sensor:right', function(contact) {
  if (beenTop) {
    this.thrusters.top(true);
  }
  else {
    this.thrusters.bottom(true);
  }
});