Magic Squares In Grid LeetCode solution in JavaScript

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var numMagicSquaresInside = function (grid) {
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  let res = 0;
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  for (let i = 0; i < grid.length - 2; i++) {
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    for (let j = 0; j < grid[0].length - 2; j++) {
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      //only check 4
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      if (
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        grid[i][j] + grid[i][j + 1] + grid[i][j + 2] == 15 &&
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        grid[i][j] + grid[i + 1][j] + grid[i + 2][j] == 15 &&
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        grid[i][j] + grid[i + 1][j + 1] + grid[i + 2][j + 2] == 15 &&
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        grid[i + 2][j] + grid[i + 2][j + 1] + grid[i + 2][j + 2] == 15
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      ) {
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        let set = new Set();
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        for (let a = i; a <= i + 2; a++) {
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          for (let b = j; b <= j + 2; b++) {
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            if (grid[a][b] >= 1 && grid[a][b] <= 9) set.add(grid[a][b]);
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          }
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        }
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        if (set.size === 9) res++;
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      }
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    }
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  }
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  return res;
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};