Battleships in a Board LeetCode solution in JavaScript

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var countBattleships = function (board) {
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  let count = 0;
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  for (let rowIndex = 0; rowIndex < board.length; rowIndex++) {
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    for (
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      let columnIndex = 0;
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      columnIndex < board[rowIndex].length;
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      columnIndex++
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    ) {
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      const isTopEmpty =
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        rowIndex === 0 || board[rowIndex - 1][columnIndex] !== "X";
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      const isLeftEmpty =
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        columnIndex === 0 || board[rowIndex][columnIndex - 1] !== "X";
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      const isBattleShip =
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        board[rowIndex][columnIndex] === "X" && isTopEmpty && isLeftEmpty;
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      if (isBattleShip) {
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        count++;
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      }
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    }
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  }
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  return count;
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};