How to obtain image size and image proportional scaling using javascript

var originImage = new Image();
function GetImageWidth(oImage) {
  if (originImage.src != oImage.src) originImage.src = oImage.src;
  return originImage.width;
}
function GetImageHeight(oImage) {
  if (originImage.src != oImage.src) originImage.src = oImage.src;
  return originImage.height;
}

function SetImage(ImgD, FitWidth, FitHeight) {
  var image = new Image();
  image.src = ImgD.src;
  if (image.width > 0 && image.height > 0) {
    if (image.width / image.height >= FitWidth / FitHeight) {
      if (image.width > FitWidth) {
        ImgD.width = FitWidth;
        ImgD.height = (image.height * FitWidth) / image.width;
      } else {
        ImgD.width = image.width;
        ImgD.height = image.height;
      }
    } else {
      if (image.height > FitHeight) {
        ImgD.height = FitHeight;
        ImgD.width = (image.width * FitHeight) / image.height;
      } else {
        ImgD.width = image.width;
        ImgD.height = image.height;
      }
    }
  }
}