Max Area of Island LeetCode solution in JavaScript

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var maxAreaOfIsland = function (grid) {
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  let result = 0;
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  const M = grid.length;
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  const N = grid[0].length;
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  const isOutGrid = (m, n) => m < 0 || m >= M || n < 0 || n >= N;
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  const island = (m, n) => grid[m][n] === 1;
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  const dfs = (m, n) => {
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    if (isOutGrid(m, n) || !island(m, n)) return 0;
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    grid[m][n] = "X";
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    const top = dfs(m - 1, n);
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    const bottom = dfs(m + 1, n);
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    const left = dfs(m, n - 1);
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    const right = dfs(m, n + 1);
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    return 1 + top + bottom + left + right;
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  };
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  for (let m = 0; m < M; m++) {
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    for (let n = 0; n < N; n++) {
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      if (!island(m, n)) continue;
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      const area = dfs(m, n);
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      result = Math.max(area, result);
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    }
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  }
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  return result;
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};