Example of how to get WebMidi input into Godot using GDScript.

WebMidi.gd

extends Node

signal move_player_to(channel, value)

signal key_fire()
signal key_slowmo()

signal key_okay()
signal key_cancel()

var strOnMidiMessage = """
function Input() {
	this.name = "midiinput"
    this.processed=false
    this.data = []
}

midiArray = new Input();

function onMIDIMessage (message) {
	/*
	var command = message.data[0];
	var note = message.data[1];
	var velocity = (message.data.length > 2) ? message.data[2] : 0; // a velocity value might not be included with a noteOff command

    switch (command) {
        case 144: // noteOn
            if (velocity > 0) {
                noteOn(note, velocity);
            } else {
                noteOff(note);
            }
            break;
        case 128: // noteOff
            noteOff(note);
            break;
        // we could easily expand this switch statement to cover other types of commands such as controllers or sysex
    }
*/
	midiArray.data.push(message)
}

function failure () {  
	alert('No access to your midi devices.') 
}

function success (midi) { 
	var inputs = midi.inputs.values(); 
	for (var input = inputs.next(); input && !input.done; input = inputs.next()) { 
		input.value.onmidimessage = onMIDIMessage; 
	} 
}

if (navigator.requestMIDIAccess) 
{ 
	navigator.requestMIDIAccess().then(success, failure); 
} else {
	alert('no midi support, you cannot play');
}"""

func _ready():

	JavaScript.eval(strOnMidiMessage, true);
	pass
	

func _process(delta):
#	# Called every frame. Delta is time since last frame.
#	# Update game logic here.
	var midiinput = _fetch_js_input()
	if midiinput.data == null:
		return
	if midiinput.data.size() == 0:
		return

	for d in midiinput.data:
		var type = d[0] & 0xf0
		var channel = d[0] & 0x0f
		if type == 0xb0:
			var cc = d[1]
			if cc == 48:
				emit_signal("move_player_to", channel, d[2])
		
		
		if (d[1] == 26):
			if d[2] == 0:
				pass
			if d[2] == 127:
				#print("key_slowmo down")
				emit_signal("key_slowmo")
				
		if (d[1] == 27):
			if d[2] == 127:
				#print("key_fire down")
				emit_signal("key_fire")
			
		if type == 0x80: # maybe note 
			emit_signal("key_fire");
			#print("Maybe Note on")
			pass
		if type == 0x90: # maybe note off
			#print("Maybe Note Off")
			pass
		pass
	pass

func _fetch_js_input():
	var input_action=JavaScript.eval("""midiArray.name""")
	var input_processed=JavaScript.eval("""midiArray.processed""")
	var number_of_messages = JavaScript.eval("""midiArray.data.length""")
	if number_of_messages == null:
		return {"action": input_action, "processed": input_processed, "data": null}
		
	var data = []
	for i in number_of_messages:
		data.append(JavaScript.eval("midiArray.data[" + str(i) + "].data"))
	JavaScript.eval("midiArray.data = []", true)
	return {"action": input_action, "processed": input_processed, "data": data}

	
func _set_js_input_as_processed():
	JavaScript.eval("""midiArray.processed=true""",true)