Rejecting promise when using fetch [duplicate]

Question

I am a bit confused with how to properly use fetch. It seems to resolve even when I get an error status back. Is the below code correct (wrapping the fetch in another promise)?

function a(url, config) {
  if (!config)
    config = {};

  config = Object.assign(config, {
    headers: {
      'content-type': 'application/json;charset=utf-8'
    }
  })
  return new Promise(
    function(resolve, reject) {
      fetch(url, config).then(
        function(res) {
          if (res.status == 200 && res.ok) {
            console.log("Promise resolved")
            resolve(res);
            return;
          }
          console.log("Promise rejected")
          reject(res);
        },
        function(rej) {
          console.log("promise rejected")
          reject(rej);
        }
      )
    }
  )
}

function b() {
  a('/test').then(
    function(res) {
      console.log('success');
      console.log(res)
    },
    function(rej) {
      console.log('Rejected');
      console.log(rej)
    }
  )
}

b();

(The above code should run fine in chrome via console... just copy/paste )

Answer 1

If you want to reject from success callback you need to do it explicitly by either returning rejected promise, e.g. return Promise.reject('error occurred'); or by throwing.

Additionally, you should not abuse Promise constructor in your case since fetch already returns promise object:

function a(url, config) {
    if (!config)
        config = {};

    config = Object.assign(config, {
        headers: {
            'content-type': 'application/json;charset=utf-8'
        }
    });

    return fetch(url, config).then(
        function(res) {
            if (res.status == 200 && res.ok) {
                console.log("Promise resolved")
                return res.json();
            }
            console.log("Promise rejected");
            throw 'promise rejected';
        },
        function(rej) {
            console.log("promise rejected");
            throw 'promise rejected';
        }
    );
}