a teeny drum machine.

drums.coffee

class DrumMachine
  constructor: (@midi) ->
    @bpm = 120
    @playing = false
    @currentStep = 0

  measure: ->
    60000 / @bpm * 4
  beat: ->
    60000 / @bpm / 4

  instruments:
    kick: 36
    hihat: 42
    clap: 39
    snare: 40
    cowbell: 56
    crash: 49

  playLoop: (pattern) ->
    @playing = true
    setInterval(=>
      thisBeat = pattern[@currentStep % 16]
      if thisBeat.length > 0
        @playNote(inst) for inst in thisBeat
      @currentStep += 1
    , @beat())

  stop: ->
    @playing = false

  playNote: (note) ->
    @midi.sendMessage [144,@instruments[note], 100]

midi = require 'midi'
out = new midi.output

try
  out.openPort 0
catch error
  out.openVirtualPort ''

dm = new DrumMachine out

console.log "Playing at #{dm.bpm} bpm, each beat is #{dm.beat()} ms"
myLoop = [['kick'],[],[],[],
          ['kick'],[],['hihat'],[],
          ['kick'],[],[],[],
          ['kick'],[],[],[]]

dm.playLoop(myLoop)