Web Audio sequencing function

seq.js

// creates a Sequence for the given AudioContext, which changes the `param` of the `node` according to a specified `seq` array with a given `smoothness`
// Example of a `seq` array:
// [ [0, 440], [1, 220], [1.5, 330], [2, 440] ]
// English: set the value to 440 immediately, then to 220 after 1 second, then to 330 after 1.5 seconds, then return to 440 after 2 seconds
//
// returns an object containing 3 methods:
//
// - `playOnce` will perform the sequence once
// - `loop` will loop the sequence indefinitely
// - `stop` will stop looping the sequence
//
// Example usage:
//
// let mySequence = sequence(
//    context,
//    oscillator,
//    'frequency',
//    [ [0, 440], [1, 220], [1.5, 330], [2, 440] ],
//    0.2
// );
//

const sequence = (ctx, node, param, seq, smoothness) => {
  const duration = seq.reduce((acc, it) => acc + it[0], 0);

  let on = true;

  const control = {
    playOnce: () => {
      for (let point of seq) {
        if (smoothness) {
          node[param].setTargetAtTime(point[1], ctx.currentTime + point[0], smoothness);
        }
        else {
          node[param].setValueAtTime(point[1], ctx.currentTime + point[0]);
        }
      };
    },
    loop: function() {
      on = true;
      node[param].cancelScheduledValues(ctx.currentTime);

      const repeat = () => {
        this.playOnce();
        if (on) this._timeout = setTimeout(repeat, duration * 1000);
      };
      this._timeout = setTimeout(repeat, duration * 1000);
    },
    stop: () => {
      clearTimeout(this._timeout);
      node[param].cancelScheduledValues(ctx.currentTime);
      on = false;
    }
  }

  return control;
}