Confused by formatting inputs example on home page

In this example [Formatting Inputs](https://github.com/bamlab/react-native-formik#formatting-inputs) I'm confused by this line:

```const formatPhoneNumber: string => string = (unformattedPhoneNumber) => ...;```

Why the double function indirection? How exactly is that adding spaces? I'm guessing it's just a sample to say that if "value" prop is a function, it'll get called but store the value untransformed.

Would love some hints.