Processing errors with Fetch API

error-handling-with-fetch.md

I really liked @tjvantoll article Handling Failed HTTP Responses With fetch(). The one thing I found annoying with it, though, is that response.statusText always returns the generic error message associated with the error code. Most APIs, however, will generally return some kind of useful, more human friendly message in the body.

Here's a modification that will capture this message. The key is that rather than throwing an error, you just throw the response and then process it in the catch block to extract the message in the body:

fetch("/api/foo")
  .then( response => {
    if (!response.ok) { throw response }
    return response.json()  //we only get here if there is no error
  })
  .then( json => {
    this.props.dispatch(doSomethingWithResult(json)) 
  })
  .catch( err => {
    err.text().then( errorMessage => {
      this.props.dispatch(displayTheError(errorMessage))
    })
  })

Frankly, I'm horrified that JavaScript let's you throw some random value, rather than an error, but hey, when in Rome...

With Express, I am doing the following

export async function fetchData(token: any, body: any) {
  const result = await fetch(
    `${config.apiUrl}/${config.projectKey}/my-request`,
    {
      method: 'POST',
      headers: {
        Authorization: token,
      },
      body: JSON.stringify(body),
    }
  );

  const json = await result.json();
  if (!result.ok) {
    throw {
      statusCode: result.status,
      ...json
    };
  }
  return json;
}

and in the route I do

router.post(
  '/some-path',
  handleErrorAsync(async (req: Request, resp: Response, _err: Errback) => {
    const data = await fetchData(req.get('authorization'), req.body);
    resp.json(data)
  })
);

And I have an error handling package that does

// async functions errors handler to avoid using try catch for every async route

export const handleErrorAsync = (func: Function) => (
  req: Request,
  res: Response,
  next: NextFunction
) => {
  func(req, res, next).catch((error: Error) => {
    next(error);
  });
};

// middleware to respond with an error when error caught
export function handleError(
  err: any,
  _req: Request,
  resp: Response,
  _next: NextFunction
) {
  if (err) {
    resp.status(err.statusCode || 500).json(err);
  }
}

And of course I add to the router

router.use(handleError);

This could be simplified with the following:

return fetch(url)
    .then(res => {
        if (!res.ok) {
            return res.json().then(json => { throw json; });
        }
        return res.json();
    })
    .catch(err => {
        // err is not a promise
    });

I found this is easier to read:

return fetch(url)
    .then(async (res) => {
        if (!res .ok) {
            throw await res.json();
        }
        return response.json();
    })
    .catch(err => {
        // err is not a promise
    });

var handlejson = async function(x){
		var t = await x.text();
		try{
			return JSON.parse(t);
		}catch(e){				
			console.log("unparsed:",t);
			return null; //or throw new Expection(e);
		}
	}
fetch(...).then(handlejson)

Oh I bet yall will love this extremely cursed wrapper I wrote for fetch a few years ago, it lets me bubble up the exact network error to the UI. With this monstrosity, I can get the actual error (e.g. ECONNREFUSED when the server isn't running!):

https://github.com/ariccio/COVID-CO2-tracker/blob/main/co2_client/src/utils/FetchHelpers.ts#L312

Actually, I think the node-fetch docs propose a nice solution for the OP ?

Instead of throwing a regular Error, they throw a custom extension of Error that contains the response. In the catch clause, they access the response:

https://github.com/node-fetch/node-fetch?tab=readme-ov-file#handling-client-and-server-errors

class HTTPResponseError extends Error {
	constructor(response) {
		super(`HTTP Error Response: ${response.status} ${response.statusText}`);
		this.response = response;
	}
}
...
	if (!response.ok) {
		throw new HTTPResponseError(response);
	}
...
catch (error) {
	console.error(error);
	const errorBody = await error.response.text();
	....
}