HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>padStart zeropad</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    function leftFillNum(num, targetLength) {
      return num.toString().padStart(targetLength, "0");
    }
    var h = 9;
    var m = 5;
    document.getElementById("out").textContent = [
      leftFillNum(123, 5),
      leftFillNum(7, 2),
      leftFillNum(42, 2),
      String(h).padStart(2, "0") + ":" + String(m).padStart(2, "0")
    ].join("\n");
  </script>
</body>
</html>

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String.padStart() — zero-pad numbers (Try It)