Count Sub Islands LeetCode solution in JavaScript

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var countSubIslands = function (grid1, grid2) {
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  const R = grid2.length,
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    C = grid2[0].length;
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  // returns no of cells in grid 2 not covered in grid1
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  function noOfNotCoveredDfs(i, j) {
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    if (i < 0 || j < 0) return 0;
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    if (i >= R || j >= C) return 0;
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    if (grid2[i][j] !== 1) return 0;
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    // mark visited
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    grid2[i][j] = 2;
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    return (
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      (grid1[i][j] === 1 ? 0 : 1) +
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      noOfNotCoveredDfs(i - 1, j) +
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      noOfNotCoveredDfs(i + 1, j) +
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      noOfNotCoveredDfs(i, j - 1) +
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      noOfNotCoveredDfs(i, j + 1)
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    );
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  }
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  let ans = 0;
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  for (let i = 0; i < R; i++) {
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    for (let j = 0; j < C; j++) {
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      if (grid2[i][j] === 1) {
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        if (noOfNotCoveredDfs(i, j) === 0) {
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          ans++;
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        }
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      }
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    }
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  }
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  return ans;
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};