Decode the Slanted Ciphertext LeetCode solution in JavaScript

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var decodeCiphertext = function (encodedText, rows) {
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  const numColumns = encodedText.length / rows;
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  const stringBuilder = [];
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  let nextCol = 1;
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  let row = 0;
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  let col = 0;
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  let index = 0;
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  while (index < encodedText.length) {
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    stringBuilder.push(encodedText[index]);
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    if (row === rows - 1 || col === numColumns - 1) {
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      row = 0;
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      col = nextCol;
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      nextCol++;
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    } else {
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      row++;
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      col++;
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    }
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    index = calcIndex(row, col, numColumns);
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  }
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  while (stringBuilder[stringBuilder.length - 1] === " ") {
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    stringBuilder.pop();
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  }
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  return stringBuilder.join("");
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};
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function calcIndex(row, col, numColumns) {
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  return row * numColumns + col;
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}