Largest Magic Square LeetCode solution in JavaScript

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var largestMagicSquare = function (grid) {
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  const row = grid.length;
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  const col = grid[0].length;
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  const startSize = row <= col ? row : col;
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  for (let s = startSize; s > 1; s--) {
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    for (let r = 0; r < grid.length - s + 1; r++) {
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      for (let c = 0; c < grid[0].length - s + 1; c++) {
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        if (isMagic(grid, r, c, s)) {
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          return s;
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        }
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      }
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    }
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  }
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  return 1;
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};
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const isMagic = (grid, i, j, size) => {
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  let targetSum = 0;
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  for (let c = j; c < j + size; c++) {
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    targetSum += grid[i][c];
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  }
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  //check rows
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  for (let r = i; r < i + size; r++) {
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    let sum = 0;
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    for (let c = j; c < j + size; c++) {
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      sum += grid[r][c];
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    }
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    if (targetSum !== sum) {
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      return false;
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    }
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  }
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  //check cols
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  for (let c = j; c < j + size; c++) {
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    let sum = 0;
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    for (let r = i; r < i + size; r++) {
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      sum += grid[r][c];
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    }
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    if (targetSum !== sum) {
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      return false;
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    }
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  }
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  //check diagonals
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  let diagSum = 0,
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    antiDiagSum = 0;
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  for (let c = 0; c < size; c++) {
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    diagSum += grid[i + c][j + c];
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    antiDiagSum += grid[i + c][j + size - 1 - c];
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  }
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  if (diagSum !== antiDiagSum || diagSum !== targetSum) {
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    return false;
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  }
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  return true;
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};