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Showing content from http://developer.mozilla.org/en-US/docs/Web/API/Web_MIDI_API below:

Web MIDI API - Web APIs

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

The Web MIDI API connects to and interacts with Musical Instrument Digital Interface (MIDI) Devices.

The interfaces deal with the practical aspects of sending and receiving MIDI messages. Therefore, the API can be used for musical and non-musical uses, with any MIDI device connected to your computer.

MIDIInputMap

Represents all of the available MIDI input ports.

MIDIOutputMap

Represents all of the available MIDI output ports.

MIDIAccess

Provides the methods to list input and output devices, and to access an individual device.

MIDIPort

Represents an individual MIDI port.

MIDIInput

Provides a method for dealing with MIDI messages from an input port.

MIDIOutput

Queues messages to the linked MIDI port. Messages can be sent immediately or after a specified delay.

MIDIMessageEvent

The event passed to the MIDIInput midimessage event.

MIDIConnectionEvent

The event passed to the MIDIAccess statechange and MIDIPort statechange events, when a port becomes available or unavailable.

Access to the API is requested using the navigator.requestMIDIAccess() method.

• The method must be called in a secure context.
• Access may be gated by the midi HTTP Permission Policy.
• The user must explicitly grant permission to use the API through a user-agent specific mechanism, or have previously granted permission. Note that if access is denied by a permission policy it cannot be granted by a user permission.

The permission status can be queried using the Permissions API method navigator.permissions.query(), passing a permission descriptor with the midi permission and (optional) sysex property:

navigator.permissions.query({ name: "midi", sysex: true }).then((result) => {
  if (result.state === "granted") {
    // Access granted.
  } else if (result.state === "prompt") {
    // Using API will prompt for permission
  }
  // Permission was denied by user prompt or permission policy
});

The navigator.requestMIDIAccess() method returns a promise that resolves to a MIDIAccess object, which can then be used to access a MIDI device. The method must be called in a secure context.

let midi = null; // global MIDIAccess object
function onMIDISuccess(midiAccess) {
  console.log("MIDI ready!");
  midi = midiAccess; // store in the global (in real usage, would probably keep in an object instance)
}

function onMIDIFailure(msg) {
  console.error(`Failed to get MIDI access - ${msg}`);
}

navigator.requestMIDIAccess().then(onMIDISuccess, onMIDIFailure);

In this example the list of input and output ports are retrieved and printed to the console.

function listInputsAndOutputs(midiAccess) {
  for (const entry of midiAccess.inputs) {
    const input = entry[1];
    console.log(
      `Input port [type:'${input.type}']` +
        ` id:'${input.id}'` +
        ` manufacturer:'${input.manufacturer}'` +
        ` name:'${input.name}'` +
        ` version:'${input.version}'`,
    );
  }

  for (const entry of midiAccess.outputs) {
    const output = entry[1];
    console.log(
      `Output port [type:'${output.type}'] id:'${output.id}' manufacturer:'${output.manufacturer}' name:'${output.name}' version:'${output.version}'`,
    );
  }
}

This example prints all MIDI input messages to the console.

function onMIDIMessage(event) {
  let str = `MIDI message received at timestamp ${event.timeStamp}[${event.data.length} bytes]: `;
  for (const character of event.data) {
    str += `0x${character.toString(16)} `;
  }
  console.log(str);
}

function startLoggingMIDIInput(midiAccess) {
  midiAccess.inputs.forEach((entry) => {
    entry.onmidimessage = onMIDIMessage;
  });
}

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