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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>substr migrate</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    function firstN(str, n) { return str.slice(0, n); }
    function fromIndex(str, i) { return str.slice(i); }
    function lastN(str, n) { return str.slice(-n); }
    document.getElementById("out").textContent = [
      firstN("CodeToFun", 4),
      fromIndex("Mozilla", 2),
      lastN("Mozilla", 3)
    ].join("\n");
  </script>
</body>
</html>

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String.substr() — migrate to slice (Try It)