We have the following call to fetch.

this.http.fetch('flasher', { method: 'post', body: jsonPayload })
    .then(response => response.json())
    .then(data => console.log(data));

This works when we receive a 200 response but logs nothing to the console when we receive a 500 response. How do we handle a 500?

Working Solution

Combining then with catch works.

fetch('http://some-site.com/api/some.json')  
  .then(function(response) {                      // first then()
      if(response.ok)
      {
        return response.text();         
      }

      throw new Error('Something went wrong.');
  })  
  .then(function(text) {                          // second then()
    console.log('Request successful', text);  
  })  
  .catch(function(error) {                        // catch
    console.log('Request failed', error);
  });

Details

fetch() returns a Promise containing a Response object. The Promise can become either fulfilled or rejected. Fulfillment runs the first then(), returns its promise, and runs the second then(). Rejection throws on the first then() and jumps to the catch().

References

MDN - Promise

MDN - Checking that the fetch was successful

Google - Introduction to Fetch