How to handle HTTP code 4xx responses in fetch api

This way we can handle all types of status accordingly.

fetch(url, {
  method: 'POST',
  headers: headers,
  body: JSON.stringify({ user_email: email }),
}).then((response) => {
  return new Promise((resolve) => response.json()
    .then((json) => resolve({
      status: response.status,
      ok: response.ok,
      json,
    })));
}).then(({ status, json, ok }) => {
  const message = json.message;
  let color = 'black';
  switch (status) {
    case 400:
      color = 'red';
      break;
    case 201:
    case 200:
      color = 'grey';
      break;
    case 500:
    default:
      handleUnexpected({ status, json, ok });
  }
})

inspiration

I'd suggest a wrapper that checks response.ok which will be true if the response code is 2xx.

Note this statement from the MDN page on fetch():

An accurate check for a successful fetch() would include checking that the promise resolved, then checking that the Response.ok property has a value of true. An HTTP status of 404 does not constitute a network error.

Here is a wrapper like this:

function fetchData() {
    return fetch.apply(null, arguments).then(response => {
         if (!response.ok) {
             // create error object and reject if not a 2xx response code
             let err = new Error("HTTP status code: " + response.status)
             err.response = response
             err.status = response.status
             throw err
         }
         return response
    })
}

The best approach I've found for this is to wrap it in a new Promise, and if response.ok is false, reject the Promise with the error context.

/**
 * Parses the JSON returned by a network request
 *
 * @param  {object} response A response from a network request
 *
 * @return {object}          The parsed JSON, status from the response
 */
function parseJSON(response) {
  return new Promise((resolve) => response.json()
    .then((json) => resolve({
      status: response.status,
      ok: response.ok,
      json,
    })));
}

/**
 * Requests a URL, returning a promise
 *
 * @param  {string} url       The URL we want to request
 * @param  {object} [options] The options we want to pass to "fetch"
 *
 * @return {Promise}           The request promise
 */
export default function request(url, options) {
  return new Promise((resolve, reject) => {
    fetch(endpoint  + url, options)
      .then(parseJSON)
      .then((response) => {
        if (response.ok) {
          return resolve(response.json);
        }
        // extract the error from the server's json
        return reject(response.json.meta.error);
      })
      .catch((error) => reject({
        networkError: error.message,
      }));
  });
}

(Top comment on https://github.com/github/fetch/issues/203)