I tried both:

var esc_regCollection = Spark.runtimeCollection("ESCReg");
    var playerIDs = [];
    
    for( var it = 0; it < esc_regCollection.count(); ++it ){
        // [] invalid
        var regPlayer = esc_regCollection[it];
        
        playerIDs.push(regPlayer._id);
    }
    // hasNext invalid
    while( esc_regCollection.hasNext()){
        
        var regPlayer = esc_regCollection.next();
        
        playerIDs.push(regPlayer._id);
    }