BeatEmitter.js

"use strict";

const events = require("events");

class BeatEmitter extends events.EventEmitter {
  constructor(transport, api) {
    super();
    this.transport = transport;
    this.api = api || global;
    this._running = false;
    this._ticks = -1;
    this._loop = this._loop.bind(this);
  }

  start() {
    if (!this._running) {
      this._running = true;
      this.api.setImmediate(this._loop);
    }
    return this;
  }

  stop() {
    this._running = false;
    return this;
  }

  _loop() {
    const beat = this.transport(this.api.Date.now() / 1000);

    if (beat.ticks !== this._ticks) {
      this._ticks = beat.ticks;
      this.emit("beat", beat);
    }
    if (this._running) {
      this.api.setImmediate(this._loop);
    }
  }
}

module.exports = BeatEmitter;

makeTransport.js

"use strict";

function bpmsec(bpm, len) {
  len = len || 4;
  return (60 / bpm) * (4 / len);
}

function makeTransport(bpm, len) {
  len = len || 4;
  const unitTicks = bpmsec(bpm, len);
  const beatsLen = len / 4;

  return (time) => {
    const ticks = Math.floor(time / unitTicks);
    const bars = Math.floor(ticks / len);
    const beats = Math.floor(ticks / beatsLen) % 4;
    const units = ticks % len;

    return { bars, beats, units, ticks };
  };
}

module.exports = makeTransport;

usage.js

"use strict";

const makeTransport = require("./makeTransport");
const BeatEmitter = require("./BeatEmitter");

const transport = makeTransport(120, 32);

const beat = new BeatEmitter(transport);

beat.on("beat", (beat) => {
  console.log(beat);
});

beat.start();