title: Fetch: set variable with fetch response and return from function
body: <p>I'm quite new with JavaScript and react.
  I have a callback from a component that gets a customer_name from a server given a id.
  The fetch works and the console.log prints the fullname correctly, but the customer_name in the last .then is not set, and the functions returns an empty string. Why is that?</p>
  
  <pre><code>// Gets the fullname of the customer from an id.
  tj_customer_name(id) {
    let customer_name = '';
  
   fetch(`/customers/${id}.json`, {
     headers: API_HEADERS,
     credentials: 'same-origin'
   })
   .then((response) =&gt; {
     if(response.ok) {
       return response.json();
     } else {
       throw new Error('Server response wasn\'t OK');
     }
   })
   .then((json) =&gt; {
     customer_name = json.first_name.concat(' ').concat(json.last_name);
     console.log(customer_name);
   });
   return customer_name;
  }
  </code></pre>