Javascript Quiz: Question 2

I wanted to convert a string to a function, so I figured this would work:

  var expression = "x * x";
  var f1 = function(x) {
    return eval(expression);
  };
  f1(10);

And it did work, but I didn't want to call eval every time I invoked f1, so I made the outer function part of the eval statement:

  var expression= "x * x";
  expression = "(function (x) { return (" + expression + "); });"
  var f2 = eval(expression);
  f2(20);

This does not work in IE but complains, "Object expected". Specifically:

{
  "name": "TypeError", 
  "message": "Object expected", 
  "number": -2146823281, 
  "description": "Object expected"
}

Question: How can this be done?

It seems that you must inline an assignment:

  var expr = "x * x";
  expr = "var f2 = (function (x) { return (" + expr + "); });"
  eval(expr);
  f2(20);

Here's a version without side effects:

function myEval(expression, context) {
  context = context || {};
  (function() {
    eval("this.expression = " + expression + ";");
  }).call(context);
  return dojo.hitch(context, "expression");
};

Usage example:

var f3 = myEval("function (x) { return ((++this.count) + (x || 0)); }", {
  count: 0
});
f3(10); // 11

UPDATE: For my particular problem, the better option was to use new Function()