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Proper way to catch errors other than network issues in fetch() call

The route of my fetch() call can yield two possible responses for which I want to do different things. But the catch() of fetch() only catches network errors if I'm not mistaken.
Below is my current code, but it looks strange to me and I have the feeling there's a more proper way to do it. If so, what is it?
fetch(`${this.baseUrl}/wp-json/contact/v1/send`, {
method: 'POST',
body: formData
})
.then((res) => {
if (res.status === 304) {
// do something
} else {
// do something different
}
})
.catch(error => {
console.log(error)
})

if you are using async await you could use the classic try...catch...finally
try {
await fetch(`${this.baseUrl}/wp-json/contact/v1/send`, {
method: 'POST',
body: formData
})
} catch (e) {
console.error(e);
} finally {
console.log('We do cleanup here');
}

How to use returned JSON error in a JavaScript async fetch call

I have an async fetch call which calls my backend to create a customer with an email address. If successful, the JSON returned is sent to a doNextThing() function.
If the backend returns a non-200 status code it also returns JSON like {"message": "Something went wrong"}. I want to catch the error and send that message to the console.
I've read dozens of slightly similar questions and edged close to an answer. So far I have the below, but if the backend's response was a 403 status code, for example, then the console outputs "FORBIDDEN". I think this is because the promise hasn't yet resolved, so doesn't yet have the full JSON response. Or something. But I can't work out what I'm missing.
async function createCustomer(email) {
return fetch("/api/create-customer", {
method: "post",
headers: {"Content-Type": "application/json"},
body: JSON.stringify({email: email})
})
.then(function(response) {
if (response.ok) {
return response.json();
} else {
return Promise.reject({
status: response.status,
statusText: response.statusText
});
}
})
.then(function(returned_data) {
doNextThing(returned_data);
})
.catch(function(e) {
console.error(e.statusText);
});
}

Personally I recommend the async/await syntax if the version you're using supports it. It really simplifies the code and allows you to easily use the try/catch syntax.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await#Browser_compatibility
It seems it doesn't work in IE though if that's a dealbreaker.
async function createCustomer(email) {
try {
const response = await fetch("/api/create-customer", {
method: "post",
headers: { "Content-Type": "application/json" },
body: JSON.stringify({ email: email })
})
if (response.ok) {
const returnedData = await response.json();
// if doNextThing is an async function, you can await it as well or just return it
doNextThing(returnedData);
} else {
throw {
json: await response.json()
status: response.status,
statusText: response.statusText
};
}
} catch (requestErr) {
// do what you like with the error
// this will be called if you "throw" above or
// if fetch() rejects
if (requestErr.json){
console.error(JSON.stringify(requestErr.json));
}
console.error("Request err:" + requestErr);
}
}
Let me know if that helps.

How to catch 401 error using fetch method of javascript

I need to catch error 401 Code of response so that I can retry after getting a new token from token endpoint. I am using fetch method get data from API.
const request: Request = new Request(url.toString(), {
headers: this.defaultRequestHeaders,
method: "get",
mode: "cors"
});
const headers: Headers = new Headers({
"Accept": "application/json",
"Content-Type": "application/json"
});
fetch(request)
.then(function(response)
{
///Logic code
})
.catch(function(error)
{
///if status code 401. Need help here
});

You can check the status and if it's not 200 (ok) throw an error
fetch("some-url")
.then(function(response)
{
if(response.status!==200)
{
throw new Error(response.status)
}
})
.catch(function(error)
{
///if status code 401...
});

Because 401 is actually a valid response to a request to a server, it will execute your valid response regardless. Only if security issues occur, or if the server is unresponsive or simply not available will the catch clause be used. Just think of it like trying to talk to somebody. Even if they say "I am currently not available" or "I don't have that information", your conversation was still successful. Only if a security guy comes in between you and stops you from talking to the recipient, or if the recipient is dead, will there be an actual failure in conversation and will you need to respond to that using a catch.
Just separate out your error handling code so you can handle it in instances that the request was successful, but does not have the desired outcome, as well as when an actual error is being thrown:
function catchError( error ){
console.log( error );
}
request.then(response => {
if( !response.ok ){
catchError( response );
} else {
... Act on a successful response here ...
}
}).catch( catchError );
I am using the response.ok suggested by #Noface in the comments, as it makes sense, but you could check for only the response.status === 401 if you want to.

You can try this
fetch(request)
.then(function(response) {
if (response.status === 401) {
// do what you need to do here
}
})
.catch(function(error) {
console.log('DO WHAT YOU WANT')
});

You can check the status of the response in then:
fetch(request)
.then(function(response) {
if (response.status === 401) {
// do what you need to do here
}
})
.catch(function(error) {});

fetch(url,{
method: 'GET',
headers,
body: JSON.stringify(aData)
}).then(response => {
if(response.ok){
return response.json();
}
return Promise.reject(response);
}).catch(e => {
if(e.status === 401){
// here you are able to do what you need
// refresh token ..., logout the user ...
console.log(e);
}
return Promise.reject(e.json());
});

(function () {
var originalFetch = fetch;
fetch = function() {
return originalFetch.apply(this, arguments).then(function(data) {
someFunctionToDoSomething();
return data;
});
};})();
source
Can one use the Fetch API as a Request Interceptor?

When you want to...
catch (error) {
console.dir(error) // error.response contains your response
}

Handling non JSON response with a Body.json() promise

I'm trying to create a scheme to intercept and handle requests from an API middleware, however, for whatever reason I'm unable to properly handle non JSON responses from my API endpoint. The following snippet works just fine for server responses formatted in JSON however say an user has an invalid token, the server returns a simple Unauthorized Access response that I'm unable to handle even though I am supplying an error callback to the json() promise. The Unauthorized Access response message is lost in the following scheme.
const callAPI = () => { fetch('http://127.0.0.1:5000/auth/', {
method: 'GET',
headers: {
'credentials': 'include',
'Accept': 'application/json',
'Content-Type': 'application/json',
'Authorization': 'Basic bXlKaGJHY2lPaUpJVXpJMU5pSXNJbVY0Y0NJNk1UUTVPRE15TVRNeU5pd2lhV0YwSWpveE5EazRNak0wT1RJMmZRLmV5SnBaQ0k2TVgwLllFdWdKNF9YM0NlWlcyR2l0SGtOZGdTNkpsRDhyRE9vZ2lkNGVvaVhiMEU6'
}
});
};
return callAPI().then(res => {
return res.json().then(responseJSON => {
if(responseJSON.status === 200){
return dispatch({
type: type[1],
data: responseJSON,
message: success
});
} else if(responseJSON.status === 401) {
return dispatch({
type: type[2],
message: responseJSON.message
});
}
return Promise.resolve(json);
}, (err) => {
console.log(err.toString(), ' an error occured');
});
}, err => {
console.log('An error occured. Please try again.');
});

Try using text method of Body: res.text().

Try to wrap your response handling code in a try...catch block like this:
return callAPI().then(res => {
try {
return res.json().then(responseJSON => {
[...]
catch(e) {
console.error(e);
}
});
Body.json() throws when the body is actually not JSON. Therefore, you should check if the body contains JSON before you call json() on it. See https://developer.mozilla.org/en-US/docs/Web/API/Response.

How to get Readable error response from JavaScript Fetch API?

I am working on Reactjs redux on front-end and Rails API as a back-end.
So now I call API with Fetch API method but the problem is I cannot get readable error message like what I got inside the network tabs
this is my function
export function create_user(user,userInfoParams={}) {
return function (dispatch) {
dispatch(update_user(user));
return fetch(deafaultUrl + '/v1/users/',
{
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
},
method: "POST",
body: JSON.stringify(userInfoParams)
})
.then(function(response) {
console.log(response);
console.log(response.body);
console.log(response.message);
console.log(response.errors);
console.log(response.json());
dispatch(update_errors(response));
if (response.status >= 400) {
throw new Error("Bad response from server");
}
})
.then(function(json){
console.log("succeed json re");
// We can dispatch many times!
// Here, we update the app state with the results of the API call.
dispatch(update_user(json));
});
}
}
But when errors came I cannot figure out how to get readable response message like I got when I check on my browser network tabs
So this is what I got from the network tabs when I got errors.
My console
This is my rails code
def create
user = User.new(user_params)
if user.save
#UserMailer.account_activation(user).deliver_now
render json: user, status: 201
else
render json: { errors: user.errors }, status: 422
end
end
But I cannot find out how can I get that inside my function

Since the text is hidden inside promise within response object, it needs to be handled like a promise to see it.
fetch(bla)
.then(res => {
if(!res.ok) {
return res.text().then(text => { throw new Error(text) })
}
else {
return res.json();
}
})
.catch(err => {
console.log('caught it!',err);
});

Similar to your answer, but with a bit more explanation... I first check if the response is ok, and then generate the error from the response.text() only for the cases that we have a successful response. Thus, network errors (which are not ok) would still generate their own error without being converted to text. Then those errors are caught in the downstream catch.
Here is my solution - I pulled the core fetch function into a wrapper function:
const fetchJSON = (...args) => {
return fetch(...args)
.then(res => {
if(res.ok) {
return res.json()
}
return res.text().then(text => {throw new Error(text)})
})
}
Then when I use it, I define how to handle my response and errors as needed at that time:
fetchJSON(url, options)
.then((json) => {
// do things with the response, like setting state:
this.setState({ something: json })
})
.catch(error => {
// do things with the error, like logging them:
console.error(error)
})

even though this is a bit old question I'm going to chime in.
In the comments above there was this answer:
const fetchJSON = (...args) => {
return fetch(...args)
.then(res => {
if(res.ok) {
return res.json()
}
return res.text().then(text => {throw new Error(text)})
})
}
Sure, you can use it, but there is one important thing to bare in mind. If you return json from the rest api looking as {error: 'Something went wrong'}, the code return res.text().then(text => {throw new Error(text)}) displayed above will certainly work, but the res.text() actually returns the string. Yeah, you guessed it! Not only will the string contain the value but also the key merged together! This leaves you with nothing but to separate it somehow. Yuck!
Therefore, I propose a different solution.
fetch(`backend.com/login`, {
method: 'POST',
body: JSON.stringify({ email, password })
})
.then(response => {
if (response.ok) return response.json();
return response.json().then(response => {throw new Error(response.error)})
})
.then(response => { ...someAdditional code })
.catch(error => reject(error.message))
So let's break the code, the first then in particular.
.then(response => {
if (response.ok) return response.json();
return response.json().then(response => {throw new Error(response.error)})
})
If the response is okay (i.e. the server returns 2xx response), it returns another promise response.json() which is processed subsequently in the next then block.
Otherwise, I will AGAIN invoke response.json() method, but will also provide it with its own then block of code. There I will throw a new error. In this case, the response in the brackets throw new Error(response.error) is a standard javascript object and therefore I'll take the error from it.
As you can see, there is also the catch block of code at the very end, where you process the newly thrown error. (error.message <-- the error is an object consisting of many fields such as name or message. I am not using name in this particular instance. You are bound to have this knowledge anyway)
Tadaaa! Hope it helps!
I've been looking around this problem and has come across this post so thought that my answer would benefit someone in the future.
Have a lovely day!
Marek

If you came to this question while trying to find the issue because response.json() throws "Unexpected token at position..." and you can't find the issue with the JSON, then you can try this, basically getting the text and then parsing it
fetch(URL)
.then(async (response) => {
if (!response.ok) {
const text = await response.text()
throw new Error(text)
}
// Here first we convert the body to text
const text = await response.text()
// You can add a console.log(text), to see the response
// Return the JSON
return JSON.parse(text)
})
.catch((error) => console.log('Error:', error))
.then((response) => console.log(response))

I think you need to do something like this
export function create_user(user,userInfoParams={}) {
return function (dispatch) {
dispatch(update_user(user));
return fetch(deafaultUrl + '/v1/users/',
{
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
},
method: "POST",
body: JSON.stringify(userInfoParams)
})
.then(function(response) {
console.log(response);
console.log(response.body);
console.log(response.message);
console.log(response.errors);
console.log(response.json());
return response.json();
})
.then(function(object){
if (object.errors) {
dispatch(update_errors(response));
throw new Error(object.errors);
} else {
console.log("succeed json re");
dispatch(update_user(json));
}
})
.catch(function(error){
this.setState({ error })
})
}
}

You can access the error message with this way:
return fetch(deafaultUrl + '/v1/users/',
{
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
},
method: "POST",
body: JSON.stringify(userInfoParams)
})
.then(function(response) {
console.log(response);
console.log(response.body);
console.log(response.message);
console.log(response.errors);
console.log(response.json());
dispatch(update_errors(response));
if (response.status >= 400) {
throw new Error("Bad response from server");
}
})
.then(function(json){
console.log("succeed json re");
// We can dispatch many times!
// Here, we update the app state with the results of the API call.
dispatch(update_user(json));
})
// here's the way to access the error message
.catch(function(error) {
console.log(error.response.data.message)
})
;

The best choice is not to catch the error in the fetch because this will be useless:
Just in your api put a response with not code error
static GetInvoicesAllData = async (req,res) =>
{
try{
let pool = await new Connection().GetConnection()
let invoiceRepository = new InvoiceRepository(pool);
let result = await invoiceRepository.GetInvoicesAllData();
res.json(result.recordset);
}catch(error){
res.send(error);
}
}
Then you just catch the error like this to show the message in front end.
fetch(process.env.REACT_APP_NodeAPI+'/Invoices/AllData')
.then(respuesta=>respuesta.json())
.then((datosRespuesta)=>{
if(datosRespuesta.originalError== undefined)
{
this.setState({datosCargados:true, facturas:datosRespuesta})
}
else{ alert("Error: " + datosRespuesta.originalError.info.message ) }
})
With this you will get what you want.

You variables coming back are not in response.body or response.message.
You need to check for the errors attribute on the response object.
if(response.errors) {
console.error(response.errors)
}
Check here https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch
You should actually be returning an error response code from the server and use the .catch() function of the fetch API

First you need to call json method on your response.
An example:
fetch(`${API_URL}`, {
method: 'post',
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
},
body: JSON.stringify(userInfoParams)
})
.then((response) => response.json())
.then((response) => console.log(response))
.catch((err) => {
console.log("error", err)
});
Let me know the console log if it didn't work for you.