Where to put utility functions in a React-Redux application?

I follow the approach of creating libraries for cases like this in my applications, and so far this works pretty well for me.

In this case, I would create a new module, e.g. {ComponentRoot}/lib/user.js which exports a function getFullName(user) or possibly getFullName(firstName, lastName, country), depending the circumstances.

Now it is just a matter of importing the module where you require the functionality, no global functions needed:

import React from 'react'
import {getFullName} from '../lib/user'

const Title = ({user}) =>
    <div>
        {getFullName(user)}
    </div>

We place our library folder on the same level as the components folder etc., but whatever works best for you should do.