BaseAudioContext.createOscillator()

// create web audio api context
var audioCtx = new (window.AudioContext || window.webkitAudioContext)();

// create Oscillator node
var oscillator = audioCtx.createOscillator();

oscillator.type = "square";
oscillator.frequency.setValueAtTime(3000, audioCtx.currentTime); // value in hertz
oscillator.connect(audioCtx.destination);
oscillator.start();