gistfile1.txt

var aliens = [{}, {}, {}];
var aliensCollidingWithPlayer = [];

for (var i = 0; i < aliens.length; i++) {
  if(colliding(alien[i], player)) { // is colliding current tick
    var collidingInPreviousTick = false;
    for (var j = 0; j < aliensCollidingWithPlayer.length; j++) {
      if (aliens[i] === aliensCollidingWithPlayer[j]) {
        collidingInPreviousTick = true;
        break;
      }
    }
    if (!collidingInPreviousTick) {
      // know that this is the first tick where we were colliding
      player.health -= 1;
      aliensCollidingWithPlayer.push(alien[i]);
    }
  } else { // not colliding in current tick
    var collidingInPreviousTick = false;
    for (var j = 0; j < aliensCollidingWithPlayer.length; j++) {
      if (aliens[i] === aliensCollidingWithPlayer[j]) {
        aliensCollidingWithPlayer.splice(i, 1);
        break;
      }
    }
  }
}



//------------------------------------------------

[a, b c]

a   a
b   b
c   c



collision: function(otherObject) {
  if (otherObject instanceof Alien) {
    //
  } else if(otherObject instanceof Pawerup) {
    //
  }
}


var objects = [{}, {}, {}];
var objectCollisions = [[obj1, obj2]];

var collisionRegisteredInPreviousTick = function(obj1, obj2) {
  var collidingInPreviousTick = false;
  for (var j = 0; j < objectCollisions.length; j++) {
    if ((obj1 === objectCollisions[i][0] && obj2 === objectCollisions[i][1])) {
      return true;
    }
  }
  return false;
};

for (var i = 0; i < objects.length; i++) {
  for (var j = i; i < objects.length; i++) {
    if (objects[i] !== objects[j]) {
      if(colliding(objects[i], objects[j])) { // is colliding current tick
        if (!collisionRegisteredInPreviousTick()) {
          if (object[i].colliding !== undefined) {
            object[i].colliding(object[j]);
          }

          if (object[i].colliding !== undefined) {
            object[j].colliding(object[i]);
          }
        }
      } else { // not colliding in current tick
        if (collisionRegisteredInPreviousTick()) {
          if (object[i].uncolliding !== undefined) {
            object[i].uncolliding(object[j]);
          }

          if (object[i].uncolliding !== undefined) {
            object[j].uncolliding(object[i]);
          }
      }
      }
    }
  }
}