assemblyscript/scripts/hexfloat.html

<style>
  html, body { margin: 0; }
  body { font-family: sans-serif; border-top: 5px solid #0074C1; }
  form { margin: 10px; }
  label { cursor: pointer; }
</style>

<script src="hexfloat.js"></script>
<form onsubmit="convert(this); return false">
  <h1>Hexadecimal float to decimal float converter</h1>
  <p>
    <label for="f64"><input id="f64" name="precision" value="f64" type="radio" checked /> f64</label>
    <label for="f32"><input id="f32" name="precision" value="f32" type="radio" /> f32</label>
    <input id="name" type="text" value="test(" />
    <button>Convert</button>
  </p>
  <p><textarea cols="120" rows="20" id="input"></textarea></p>
  <p><textarea cols="120" rows="20" id="output" readonly></textarea></p>
</form>

<script>
function convert(form) {
  var isF64 = document.getElementById("f64").checked;
  var name = document.getElementById("name").value;
  var input = document.getElementById("input").value;
  document.getElementById("output").value = input
    .replace(/\b(\-?0x[0-9a-fA-F]*(?:\.[0-9a-fA-F]+)?[pP][+-]?[0-9]+\b)/g, ($0, $1) => {
      var val = parse($1);
      return val.toPrecision(isF64 ? 18 : 10);
    })
    .replace(/\bnan\b/g, "NaN")
    .replace(/\inf\b/g, "Infinity")
    .replace(/^T\(RN, */mg, name + "(")
    .replace(/\)$/mg, ");")
    .replace(/ +/g, " ");
}
</script>