Um-sequencer NPM

um-sequencer

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

Installation

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

Example

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)
  }
}

API

Sequencer(getCurrentTime, options)

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.

sequencer.play(events, options)

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.
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.