Be careful about using `use strict` outside functions, as that will set the entire page in strict mode, which might break dependencies, that are not developed for strict mode.

Instead, wrap your application/modules in functions, and use `use strict` inside them.

```javascript
(function(){
    'use strict';
    
    document.addEventListener("DOMContentLoaded", function () {

        var createForm = document.querySelector("form.upload.create")
        if (createForm) { Upload.creatable(createForm) }
     });
}());
```

A good linter (jslint, jshint) will capture such things. I suggest using one on save in your editor and probably also as a pre-commit hook.