Every array in javascript has a length property. you could compare the two and loop the array with the bigger length.

var length1 = array1.length,
 length 2 = array2.length;

if( length1 > length2 ){
    for(var i = 0 ; i < length1; i++){
     // stuff if array 1 is bigger
   }
}else if(length2 > length1){
  for(var j = 0; j< length2; j++){
  // stuff if array 2 is bigger
 }else{
  //if equal
}

}