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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>substr compare</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var str = "01234";
    var a = 1;
    var l = -2;
    document.getElementById("out").textContent = [
      JSON.stringify(str.substr(a, l)),
      str.slice(a, a + l),
      str.substring(a, a + l)
    ].join("\n");
  </script>
</body>
</html>

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String.substr() — vs slice / substring (Try It)