I was confused about the intention of this method, it would be clearer to be explicit about why you're stashing the created instance. I'm exaggerating a bit here, but maybe something like:

```
    function init_constructor(constructor, params) {
      var instance = new constructor(params);
      stashInstanceOfInCaseTheTestsNeedIt(constructor, instance);
      return instance;
    }

   function stashInstanceOfInCaseTheTestsNeedIt() {
      GOVUK.instances[constructor.name] = GOVUK.instances[constructor.name] || [];
      GOVUK.instances[constructor.name].push(instance);
   }
```