example.js

function example01(audioContext, pulse) {
  var destination = audioContext.destination;
  var t0 = audioContext.currentTime;
  
  pulse(destination, t0, { frequency: 440, volume: 0.3, duty: 0.8 });
}

pulse.js

function pulse(destination, playbackTime, opts) {
  var t0 = playbackTime;
  var audioContext = destination.context;
  var oscillator = audioContext.createOscillator();
  var waveShaper = audioContext.createWaveShaper();
  var gain = audioContext.createGain();
  var frequency = opts.frequency;
  var volume = opts.volume;
  
  var curve = new Float32Array(2048);
  
  for (var i = 0; i < 2048; i++) {
    curve[i] = (i / 2048) < 1 - opts.duty ? -1 : +1;
  }

  oscillator.type = "triangle";
  oscillator.frequency.value = frequency;
  oscillator.start(t0);
  oscillator.connect(waveShaper);
  
  waveShaper.curve = curve;
  waveShaper.connect(gain);

  gain.gain.value = volume;
  gain.connect(destination);
}