measure width in px chrome extension

var fromX, fromY;
var svg = document.createElementNS ('http://www.w3.org/2000/svg',"svg");
svg.setAttribute("style", "position: absolute; top:0;left:0;height: " + document.body.clientHeight + "px;width: 100%");
var line = document.createElementNS('http://www.w3.org/2000/svg','line');
line.setAttribute("style", "stroke-width: 4; stroke: red");

svg.appendChild(line);
document.body.appendChild(svg);

document.body.addEventListener("mousedown", function (e) {
  fromX = e.pageX;
  fromY = e.pageY;
});

document.body.addEventListener("mousemove", function (e) {
  if (fromX === undefined) {
    return;
  }

  line.setAttribute("x1", fromX);
  line.setAttribute("x2", e.pageX);
  line.setAttribute("y1", fromY);
  line.setAttribute("y2", e.pageY);

  console.log(
    [fromX, fromY], " to ", [e.pageX, e.pageY], "Distance:",
    Math.sqrt(Math.pow(fromX - e.pageX, 2) + Math.pow(fromY - e.pageY, 2))
  );
});

document.body.addEventListener("mouseup", function (e) {
  fromX = undefined;
  fromY = undefined;
});