Calculate the new size of an element w*h with a new width or height.

CalculateNewSizes.js

(function CalculateNewSizes(){
  var width, height, nWidth, nHeight, tmp;
  tmp = prompt('Current dimensions 123x1234: ').split('x');
  width = tmp[0];
  height = tmp[1];
  tmp = prompt('Set a new value for width. (prefix h for new Height)').replace(/ /g, '');
  if(tmp.indexOf('h') !== -1) {
    nHeight = tmp.replace(/h/, '');
    nWidth = parseInt(width*nHeight/height);
  } else {
    nWidth = tmp;
    nHeight = parseInt(height*nWidth/width);
  }
  alert('new width: ' + nWidth + "\n" + 'new height: ' + nHeight);
}());