var arr = [1,2, -3, 15, 77, 12, 55];
var highest = 0, secondHighest = 0;
// OR var highest = arr[0], secondHighest = arr[0];

for(var i=0; i highest){
    secondHighest = highest;
    highest = arr[i]; 
  }

  if(arr[i] < highest && arr[i] > secondHighest){
    secondHighest = arr[i];
  }
}

console.log('>> highest number : ',highest); // 77
console.log('>> secondHighest number : ',secondHighest); // 55