`DEFAULT_FORMATTERS` is a class level structure, and `optFieldFormatters` is an instance level structure. We could use prototype chain here to make sure that an instance is not going to modify the default formatters:

```
this.formatters = optFieldFormatters || {};
this.formatters.__proto__ = DEFAULT_FORMATTERS;
```

the rest is not needed. In the case of the `custom_` this is really not necessary because that type does not exists.