A simple and generic JavaScript music sequencer library with no dependencies.

yarn add um-sequencer or npm install um-sequencer --save

import Sequencer from 'um-sequencer'

let audioContext = new window.AudioContext()
let sequencer = Sequencer(() => audioContext.currentTime)

sequencer.play([
  { time:   0, callback: Beep(440) },
  { time: 1/4, callback: Beep(440) },
  { time: 1/2, callback: Beep(440) },
  { time: 3/4, callback: Beep(880) }
], {
  tempo: 140,
  loopLength: 1
})

setTimeout(sequencer.stop, 5000)

function Beep(freq) {
  return (time) => {
    let osc = audioContext.createOscillator()
    osc.frequency.value = freq
    osc.connect(audioContext.destination)
    osc.start(time)
    osc.stop(time + 0.1)
  }
}

Creates a sequencer object with the given clock source & timing options.

getCurrentTime is a function which should return the current time in seconds. The most typical source for this would be the Web Audio API AudioContext currentTime property, but it could be any high-resolution clock.

options is a config object which may include:

• interval, the time between clock ticks in seconds. Defaults to 0.025.
• lookahead, the window of time after a tick in which found events will be scheduled. Defaults to 0.1.
• useWorker, whether or not to run the clock in a web worker. Defaults to true.

If you do not understand these options, don't mess with them. For more information see A Tale Of Two Clocks.

Begins playback of a sequence of events with the given options.

events is an array of event objects each of which must include two attributes:

• time, the musical time in whole notes at which the event should fire.
• callback, the function to be called at the given time.

options is a config object which may include:

• tempo, the tempo in beats per minute. Defaults to 120bpm.
• startTime, the time (according to getCurrentTime) at which to begin playing the sequence. Defaults to immediately.
• loopLength, a length in whole notes at which to loop the sequence. If omitted, the sequence will not loop.
• onStop, a callback which will be called when the sequence stops playback. The callback will be passed a string, either 'stopped' if the stop() method was called or 'finished' if the end of the sequence was reached and no loopLength was specified.

Changes the tempo of the sequence during playback. Note that this method has no effect if called while the sequencer is stopped.

Returns a boolean indicating whether or not the sequencer is playing.

Stops playback of the sequence.