Трябва да се пази най-добрия играч до момента и при обхождането на входните данни да се ъпдейтва, ако открием по-добър такъв.

Примерно решение:

function bestPlayer(input) {
  let i = 0;
  let command = input[i];
  i++;

  let bestPlayerName = '';
  let bestPlayerGoals = Number.MIN_SAFE_INTEGER;

  while (command !== 'END') {
    let currentPlayerName = command;
    let currentPlayerGoals = Number(input[i]);
    i++;

    if (currentPlayerGoals > bestPlayerGoals) {
      bestPlayerName = currentPlayerName;
      bestPlayerGoals = currentPlayerGoals;
    }

    if (currentPlayerGoals >= 10) {
      break;
    }

    command = input[i];
    i++;
  }

  console.log(`${bestPlayerName} is the best player!`);
  if (bestPlayerGoals >= 3) {
    console.log(`He has scored ${bestPlayerGoals} goals and made a hat-trick !!!`);
  } else {
    console.log(`He has scored ${bestPlayerGoals} goals.`);
  }
}

Макар и късно ето едно решение и от мен, което доста прилича на това на колегата, но все пак се различава.

function bestPlr(input) {
    let index = 0;

    let command = input[index];
    index++;

    let maxGoals = 0;
    let playerWinner = "";

    let isThereHatTrick = false;

    while (command !== "END") {
        
        let currentPlayer = command;

        let currentGoals = Number(input[index]);
        index++;

        if (currentGoals >= 10) {
            maxGoals = currentGoals;
            playerWinner = currentPlayer;
            break;
        };

        if (currentGoals > maxGoals) {
            maxGoals = currentGoals;
            playerWinner = currentPlayer;
        }

            if (maxGoals >= 3) {
            isThereHatTrick = true;
        }

        command = input[index];
        index++;
    }

    console.log(`${playerWinner} is the best player!`);

    if (isThereHatTrick) {
        console.log(`He has scored ${maxGoals} goals and made a hat-trick !!!`);
    } else {
        console.log(`He has scored ${maxGoals} goals.`)
    }
}