存在捕获时可能未处理的承诺被拒绝

Question

I have got the following code:

export function fetchValueFromApi(){
    return function act(dispatch){
        dispatch(fetchingLimit);
        return fetch('https://someapi/v1/foo/bar?api_key=123456789')
          .then(response =>{
              console.log("response",response.json());
              response.json();
          }).then(json =>{
              if (json.data()) {
                  dispatch(receivingValue(json.data.result))
              } else {
                  throw new Error('Error');
              }
          }).catch(error => dispatch(receivingValueFailed(error)))
    }
}

Now I know this call won't succeed. I am expecting it to fail and go into the catch. However, I am getting this error:

Possible Unhandled Promise Rejection

So for some reason we're not hitting the .catch .

How could I solve this?

Answer 1

So you are hitting the catch, just not with the error you expected.

The 403 is not an error as far as fetch is concerned, since the request itself was made successfully (the response is just not what your application expected). You have to handle 40X errors yourself. And as your console.log reveals, the exception happens before your throw new Error is reached.

A fetch() promise will reject with a TypeError when a network error is encountered or CORS is misconfigured on the server side, although this usually means permission issues or similar — a 404 does not constitute a network error, for example.

From https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch

You should

1. return response.json() in the first .then handler: .then(response => response.ok && response.json())
2. Add a safer check in the second .then handler like if (json && json.data)
3. dispatch the failure action instead of throwing an error if there is no json data

Answer 2

You're not return ing the promises from your then handlers, so there is no chaining. The response body is not even awaited. The catch handler is not chained to the promise that actually rejects, therefore it the error is indeed unhandled.

export function fetchValueFromApi(){
    return function act(dispatch){
        dispatch(fetchingLimit);
        return fetch('https://someapi/v1/foo/bar?api_key=123456789')
        .then(response => {
            var body = response.json();
            console.log("response body", body);
            return body;
//          ^^^^^^
        }).then(json => {
            if (json.data ) {
//                       ^
                return dispatch(receivingValue(json.data.result))
//              ^^^^^^
              } else {
                  throw new Error('Error');
              }
          }).catch(error =>
              dispatch(receivingValueFailed(error))
          )
     }
}

Remember that arrow functions only implicitly return the expression value when you use the concise body syntax, ie without block braces.