Count Servers that Communicate LeetCode solution in JavaScript

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var countServers = function (grid) {
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  const m = grid.length;
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  const n = grid[0].length;
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  const lastRows = [];
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  const lastCols = [];
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  const set = new Set();
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  for (let i = 0; i < m; i++) {
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    for (let j = 0; j < n; j++) {
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      if (grid[i][j] === 1) {
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        const currIdx = i * n + j;
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        const rowNeiIdx = lastRows[i];
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        if (rowNeiIdx != null) {
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          set.add(currIdx).add(rowNeiIdx);
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        }
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        const colNeiIdx = lastCols[j];
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        if (colNeiIdx != null) {
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          set.add(currIdx).add(colNeiIdx);
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        }
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        lastRows[i] = currIdx;
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        lastCols[j] = currIdx;
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      }
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    }
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  }
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  return set.size;
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};