fetch should throw an error when it receives a 404

[tobireif]

When there's eg a 404, fetch should throw an error, so that I can .catch() and handle it. (The error related code in the readme should be in fetch by default.)

[mislav]

You would think so, but that's not per spec. The spec only rejects the promise if there was an error making or receiving the response. A HTTP 404 should be treated as a "successful" response and is up to the user to decide what to do with such response.

If you want to reject non-20x responses, you can make your own wrapper:

function myFetch(url, options) {
  if (options == null) options = {}
  if (options.credentials == null) options.credentials = 'same-origin'
  return fetch(url, options).then(function(response) {
    if (response.status >= 200 && response.status < 300) {
      return Promise.resolve(response)
    } else {
      var error = new Error(response.statusText || response.status)
      error.response = response
      return Promise.reject(error)
    }
  })
}

This resembles XMLHttpRequest in its handling of same-domain cookies and successful responses.

[tobireif]

Actually, in this specific case, I want to accept only status 200, so having my own code "if (response.status === 200) ..." instead of "if (response.status >= 200 && response.status < 300) ..." is handy.

Thanks for the wrapper code, but for my project, the following is sufficient AFAICS:

function fetchStatusHandler(response) {
  if (response.status === 200) {
    return response;
  } else {
    throw new Error(response.statusText);
  }
}

(Inside a Promise wrapper containing additional stuff:)

fetch(url)
  .then(
    fetchStatusHandler
  ).then(function(response) {
    return response.blob();
  }).then(function(blob) {
    resolve(blob.size);
  }).catch(function(error) {
    reject(error);
  });

Also: related: https://github.com/whatwg/fetch/issues/60 .