Diagonal Traverse LeetCode solution in JavaScript

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var findDiagonalOrder = function (matrix) {
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  const res = [];
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  for (
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    let r = 0,
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      c = 0,
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      d = 1,
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      i = 0,
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      len = matrix.length * (matrix[0] || []).length;
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    i < len;
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    i++
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  ) {
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    res.push(matrix[r][c]);
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    r -= d;
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    c += d;
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    if (!matrix[r] || matrix[r][c] === undefined) {
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      // We've fallen off the...
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      if (d === 1) {
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        if (matrix[r + 1] && matrix[r + 1][c] !== undefined) {
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          // ...top cliff
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          r++;
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        } else {
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          // ...right cliff
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          r += 2;
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          c--;
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        }
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      } else {
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        if (matrix[r] && matrix[r][c + 1] !== undefined) {
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          // ...left cliff
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          c++;
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        } else {
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          // ...bottom cliff
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          r--;
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          c += 2;
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        }
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      }
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      d = -d;
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    }
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  }
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  return res;
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};