The Blind Prophet challenge (http://theblindprophet.com)

prophet.js

var lastDist = null;

function initY(distance) {
  actionFunc = setYDir;
  return 'up';
}

function setYDir(distance) {
  var yDir = lastDist <= distance ? 'down' : 'up';
  actionFunc = moveY(yDir);
  return yDir;
}

function moveY(dir) {
  return function(distance) {
    if(lastDist == distance) {
      actionFunc = initX;
      return 'right';
    } else if(lastDist < distance) {
      actionFunc = initX;
      return dir == 'up' ? 'down' : 'up';
    }
    return dir;
  };
}

function initX(distance) {
  actionFunc = setXDir;
  return 'right';
}

function setXDir(distance) {
  var xDir = lastDist <= distance ? 'left' : 'right';
  actionFunc = moveX(xDir);
  return xDir;
}

function moveX(dir) {
  return function(distance) {
    return dir;
  };
}

var actionFunc = initY;

function tick(distance) {
  var dir = actionFunc(distance);
  lastDist = distance;
  return dir;
}