simple-instrument.ts

const BASE_HELTZ = 440;

function generateId() {
  return Math.random()
    .toString(36)
    .substr(2, 9);
}
function noteToHz(note: number) {
  return BASE_HELTZ * 2 ** ((note - 69) / 12);
}

interface Instrument {
  id: string;
  noteOn(note: number, duration?: number): void;
  noteOff(note: number): void;
}
class SimpleInstrument implements Instrument {
  id: string;
  private gainNode: GainNode;
  private oscillators = new Map<number, OscillatorNode>();
  constructor(private audioContext: AudioContext) {
    this.id = generateId();
    this.gainNode = audioContext.createGain();
    this.gainNode.gain.value = 0.01;
  }
  createOscillator(note: number): OscillatorNode {
    const oscillator = this.audioContext.createOscillator();
    oscillator.type = "square";
    oscillator.frequency.value = noteToHz(note);
    oscillator.connect(this.gainNode);
    this.gainNode.connect(this.audioContext.destination);
    return oscillator;
  }
  cleanupOscillator(note: number, oscillator: OscillatorNode): void {
    oscillator.disconnect();
    this.oscillators.delete(note);
  }
  noteOn(note: number, durationInSeconds?: number) {
    this.noteOff(note);
    const oscillator = this.createOscillator(note);
    oscillator.addEventListener("ended", () => {
      this.cleanupOscillator(note, oscillator);
    });
    try {
      oscillator.start();
      this.oscillators.set(note, oscillator);
      if (durationInSeconds) {
        oscillator.stop(this.audioContext.currentTime + durationInSeconds);
      }
    } catch (e) {
      oscillator.stop();
    }
  }
  noteOff(note: number) {
    const oscillator = this.oscillators.get(note);
    if (oscillator) {
      oscillator.stop();
    }
  }
}
    
const audioCtx = new AudioContext();
const simple = new SimpleInstrument(audioCtx);