Creating multiple levels of json from loop-Collection of common programming errors

I’m writing a directive in angular to help validate fields. It is working up to the point of storing the result of the validation.

I want to store it in an object like error.password.old

the code to do this looks like this:

if(!(typeof iAttrs.ngModel == "undefined"))
{
    var model = iAttrs.ngModel.split('.');
    var length = model.length;
    var target = scope.error;

    if(typeof validation[iAttrs.validate] == "function")
    {
       for(var index = 0; index < length; index++)
       {
           target = target[model[index]];
       }
       target = validation[iAttrs.validate]();
       scope.$apply();
    }
    else throw("function " + iAttrs.validate + " does not exist.");
 }

iAttrs.ngModel is holds “password.old”.

Google is currently throwing:

Uncaught TypeError: Cannot read property ‘old’ of undefined

This is thrown on the second iteration of the for loop.

I understand that it is undefined but i need to find a way to define it on the fly without the normal {} notation.