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var enemies = Players.enemies; // array of enemy players objects

Players.enemies.forEach(function(enemy) {
  // enemy = players object for current enemy
  // enemy.id = id of enemy player
});

// you could do this:
var isEnemy = !allianceExistsBetween(me, playerID);
 
// but it won't filter out spectators or vacant slots
 
// this, on the other hand, does filter them out:
var isEnemy = Players[playerID].isEnemy;
 
// if you want to know if they are living enemy:
var isLivingEnemy = Players[playerID].isEnemy && Players[playerID].isAlive;