Can you dynamically add local variables to a function?

I'm pretty sure eval is your only answer; but you need to be aware that if there's any input outside of your control involved, it isn't safe

function dynamicArgs (varName, varValue) {
  eval("var " + varName + "=" + JSON.encode(varValue) );
  alert(a);
}

dynamicArgs("a", "value");

You can see the problem with this. How is your function supposed to call the dynamic variable if it doesn't know its name? I hardcoded it to the a variable since I pass it in when calling it, but that's not a good solution. The only solution would be another eval. You really should think about what you need to do and whether this is useful. But it's doable.

Here it is in action: http://jsfiddle.net/mendesjuan/GG3Wu/

function dynamicArgs (varName, varValue) {
  eval('var ' + varName + "='" + varValue + "';");
  alert(eval(varName));
}

dynamicArgs("f", "Here I am");

Now here's an example like what you're doing, creating a variable from this.MyConstructor http://jsfiddle.net/mendesjuan/AK3WD/

var ns = {
    MyConstructor: function(val) {
       this.prop = val;
    },

    runConstructor: function(val) {
      var Ctor = "MyConstructor";
      eval('var ' + Ctor + ' =  this.' + Ctor);
      return new MyConstructor(val);
    }
}


alert( ns.runConstructor("Hello").prop );

And here's an example if you wanted to import all the values from an object into the scope;

http://jsfiddle.net/mendesjuan/AK3WD/1/

var ns = {
    MyConstructor: function(val) {
       this.val= val;
    },

    anotherProperty: 5,

    runConstructor: function(val) {
        // Bring all the variables from this into this scope
        for (var prop in this) {
            eval('var ' + prop + ' =  this.' + prop);
        }
        alert('Testing var anotherProperty: ' + anotherProperty);
        var obj =  new MyConstructor(val);
        alert('Created MyConstructor: its prop is ' + obj.val)
    }
}


ns.runConstructor("Hello");

There is controversial with, which has some great applications, but is marginally slow and prone to errors. It throws an error in the strict mode (which you should always opt into) and is going to be deprecated.

var sampleObject = {
  someFunction: function() {},
  b: 10
}

with (sampleObject) {
    typeof someFunction // "function"

    var a = 42
    var b = 20
}

sampleObject.a // undefined
sampleObject.b // 20

Note, that new variables defined in a with-block won't be added to the object. Nevertheless, if the object already had an eponymous property in it, this property would be modified (thanks, @Rocket).

Just for fun, here's an implementation of extract using eval (which is even more evil than with). You can do unspeakable things with it, for example if your object has properties like sampleObject['x; while (true) { alert("Hi!") }'].

This is how I did it:

   function smObject ( object) {
    return function () {
        function getter(prop) {
            return function() {
                return this[prop];
            }
        }

        function setter(prop) {
            return function(data) {
                this[prop]=data;
            }
        }

        for (var o = 0; o < object.length; o++) {
            this[object[o]] = {};
            this['get' + object[o]] = getter(object[o]);
            this['set' + object[o]] = setter(object[o]);
        }
    }
}

now you can instantiate a function like this:

var fields = ['Name', 'Id', 'Other', '....' ]
var MyFunction = smObject( fields );
var myObject = new MyFunction();

// getter/setters
myObject.setId(5);
myObject.getId(); // will return 5

Regards, Emanouil