var ctx = document.querySelector("canvas").getContext("2d"),
    rng = document.querySelector("input"),
    val = document.querySelector("span"),
    data = [], scale = 30;

// generate sinus wave with noise jitters
for(var i = 0; i < ctx.canvas.width; i += 2)
    data.push(Math.sin(i*0.1) * Math.random() + Math.random())

// draw initial smoothed curve (length=1, no smoothing)
drawWMA();

// calculate moving average
function drawWMA() {
    var len = +rng.value,        // get smoothing length (number of previous points)
        dataa = [], datab = [],  // pass A and B arrays
        weight = 0;              // calc weight based on length
  
  val.innerHTML = len;
  
  ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
  ctx.beginPath();
  
  // calc weight
  for(var i = 1; i <= len; i++) weight += i;  // add range together [1, length]
  
  // plot original data at top of canvas
  plot(data, 30);

  // PASS 1: Calc new smoothed array
  dataa = calcWMA(data, len, weight);
  
  // plot smoothed curve
  ctx.fillText("FIRST PASS:", 0, 100);
  plot(dataa, 120);
    
  // PASS 2 (optional)
  datab = calcWMA(dataa, len, weight);
  ctx.fillText("SECOND PASS:", 0, 190);
  plot(datab, 210);
  
  ctx.stroke();  // render plots
}

function calcWMA(data, len, weight) {
  var i, t, datao = [];
  
  // calc new smoothed array 
  for(i = 0; i < data.length; i++) {       // iterate from length to end of data
    var v = 0;                             // calc average value for this position
    for(t = 0; t < len; t++) {             // [1, len]
      if (i-t >= 0)
        v += data[i-t] * ((t+1) / weight); // weight previous values based on -delta
    }
    datao.push(v);                         // store new value
  }
  return datao
}

function plot(data, y) {
  ctx.moveTo(0, y + data[0]*scale);
  for(i = 1; i < data.length; i++) ctx.lineTo(i * 2, y + data[i]*scale);
}

rng.onchange = rng.oninput = drawWMA;

<label>Points to consider: <input type="range" min=1 max=50 value=1></label><span>1</span><br>
<canvas width=600 height=300></canvas>