[xelawho]

if you wanted to avoid the much-despised eval...

var str=function(s){
return eval(s.replace(/x/gi,"*"))
}

alert(str("14X10+2X1"))

you could make a dynamic script...

var str = function (s){
  var scr = document.createElement("script");
  scr.type = "text/javascript";
  scr.text = "ans = " + s.replace(/x/gi,"*") + ";";
  document.body.appendChild(scr);
  document.body.removeChild(scr);
  return ans;
}

alert(str("14X10+2X1"))

although I suspect they amount to much the same thing...