How to write this JavaScript code without eval?

var typeOfString = eval("typeof " + that.modules[modName].varName);
if (typeOfString !== "undefined") {
  doSomething();
}

The point is that the name of the var that I want to check for is in a string.

Maybe it is simple but I don't know how.

Edit: Thank you for the very interesting answers so far. I will follow your suggestions and integrate this into my code and do some testing and report. Could take a while.

Edit2: I had another look at the could and maybe itis better I show you a bigger picture. I am greatful for the experts to explain so beautiful, it is better with more code:

MYNAMESPACE.Loader = ( function() {

  function C() {
    this.modules = {};
    this.required = {};
    this.waitCount = 0;
    this.appendUrl = '';
    this.docHead = document.getElementsByTagName('head')[0];
  }

  function insert() {
    var that = this;
    //insert all script tags to the head now!
    //loop over all modules:
    for (var modName in this.required) {
      if(this.required.hasOwnProperty(modName)){
        if (this.required[modName] === 'required') {
          this.required[modName] = 'loading';
          this.waitCount = this.waitCount + 1;
          this.insertModule(modName);
        }
      }
    }

    //now poll until everything is loaded or 
    //until timout

    this.intervalId = 0;

    var checkFunction = function() {
      if (that.waitCount === 0) {
        clearInterval(that.intervalId);
        that.onSuccess();
        return;
      }
      for (var modName in that.required) {
        if(that.required.hasOwnProperty(modName)){
          if (that.required[modName] === 'loading') {
            var typeOfString = eval("typeof " + that.modules[modName].varName);
            if (typeOfString !== "undefined") {
              //module is loaded!
              that.required[modName] = 'ok';
              that.waitCount = that.waitCount - 1; 
              if (that.waitCount === 0) {
                clearInterval(that.intervalId);
                that.onSuccess();
                return;
              }
            }
          }
        }
      }
    };

    //execute the function twice a second to check if all is loaded:
    this.intervalId = setInterval(checkFunction, 500);
    //further execution will be in checkFunction,
    //so nothing left to do here
  }
  C.prototype.insert = insert;

  //there are more functions here...

  return C;
}());


var myLoader = new MYNAMESPACE.Loader();

//some more lines here... 

myLoader.insert();