frequencyBinCount

HTML

<body>
  <h1>Oscillator</h1>
  <canvas class="visualizer";id="myCanvas";width="640" height="100"></canvas>
</body>

CSS

div {
 display: inline;
}

JavaScript

// Create the Audio Context

var context = new AudioContext();
var analyser = context.createAnalyser();
var WIDTH = 300;
var HEIGHT = 300;

// Create your oscillator, filter and gain node by declaring them as variables

var osc = context.createOscillator();

osc.frequency.value = 500;

// Connect the nodes together

function makeConnection() {
    osc.connect(analyser);
}

// Play the sound inside of Chrome

function playSound() {
    analyser.connect(context.destination);
    osc.start(0);
    osc.stop(100);
}

makeConnection();
playSound();

var canvas = document.querySelector('.visualizer');
var myCanvas = canvas.getContext("2d");

analyser.fftSize = 2048;
var bufferLength = analyser.frequencyBinCount; //an unsigned long value half that of the FFT size. This generally equates to the number of data values you will have to play with for the visualization
var dataArray = new Uint8Array(bufferLength);

analyser.getByteTimeDomainData(dataArray); 

console.log(dataArray);

myCanvas.clearRect(0, 0, WIDTH, HEIGHT);

function draw() {
  drawVisual = requestAnimationFrame(draw);
  analyser.getByteTimeDomainData(dataArray);
  
  myCanvas.fillStyle = 'rgb(200, 200, 200)';
  myCanvas.fillRect(0, 0, WIDTH, HEIGHT);
  myCanvas.lineWidth = 2;
      myCanvas.strokeStyle = 'rgb(0, 0, 0)';

      myCanvas.beginPath();
  var sliceWidth = WIDTH * 1.0 / bufferLength;
      var x = 0;
  
  for(var i = 0; i < bufferLength; i++) {
   
        var v = dataArray[i] / 128.0;
        var y = v * HEIGHT/2;

        if(i === 0) {
          myCanvas.moveTo(x, y);
        } else {
          myCanvas.lineTo(x, y);
        }

        x += sliceWidth;
      };
  
  myCanvas.lineTo(canvas.width, canvas.height/2);
      myCanvas.stroke();
    };

draw();