HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>requestMIDIAccess — list</title>
</head>
<body>
  <button id="btn" type="button">List MIDI ports</button>
  <pre id="out">Click to request access and list ports.</pre>
  <script>
    async function listMidiPorts() {
      var access = await navigator.requestMIDIAccess();
      var inputNames = [];
      var outputNames = [];
      for (var input of access.inputs.values()) {
        inputNames.push(input.name || "(unnamed input)");
      }
      for (var output of access.outputs.values()) {
        outputNames.push(output.name || "(unnamed output)");
      }
      return JSON.stringify({ inputNames: inputNames, outputNames: outputNames });
    }
    document.getElementById("btn").addEventListener("click", function () {
      listMidiPorts()
        .then(function (msg) {
          document.getElementById("out").textContent = msg;
        })
        .catch(function (err) {
          document.getElementById("out").textContent = "Error: " + err.name;
        });
    });
  </script>
</body>
</html>

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requestMIDIAccess — list ports (Try It)