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const getScore = (board, mark) => {
    const playerOneTurns = board.reduce((array, element, index) => {
      if (element === 'X') return index
      return array;
    }, []);

which can be made even shorter:

const getScore = (board, mark) => {
    const playerOneTurns = board.reduce((array, element, index) => {
      return element === 'X'
      ? index
      : array
    }, []);

This basically reads as if element === 'X' eveluates to TRUE then return index, otherwise instead return array.