AnalyserNode.getFloatTimeDomainData()

var audioCtx = new AudioContext();
var analyser = audioCtx.createAnalyser();
var dataArray = new Float32Array(analyser.fftSize); // Float32Array needs to be the same length as the fftSize
analyser.getFloatTimeDomainData(dataArray); // fill the Float32Array with data returned from getFloatTimeDomainData()

var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var analyser = audioCtx.createAnalyser();

  ...

analyser.fftSize = 1024;
var bufferLength = analyser.fftSize;
console.log(bufferLength);
var dataArray = new Float32Array(bufferLength);

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

function draw() {
  drawVisual = requestAnimationFrame(draw);
  analyser.getFloatTimeDomainData(dataArray);

  canvasCtx.fillStyle = 'rgb(200, 200, 200)';
  canvasCtx.fillRect(0, 0, WIDTH, HEIGHT);
  canvasCtx.lineWidth = 2;
  canvasCtx.strokeStyle = 'rgb(0, 0, 0)';
  canvasCtx.beginPath();

  var sliceWidth = WIDTH * 1.0 / bufferLength;
  var x = 0;

  for(var i = 0; i < bufferLength; i++) {
    var v = dataArray[i] * 200.0;
    var y = HEIGHT/2 + v;

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

  canvasCtx.lineTo(canvas.width, canvas.height/2);
  canvasCtx.stroke();
};

draw();