How to handle failed ajax request with promise

Question

I am making multiple ajax requests like this

imgPromises = [];

imgPromise1 = $.ajax({
  url: s3Url,
  type: "POST",
  data: s3FormData,
  mimeType: "multipart/form-data",
  contentType: false,
  cache: false,
  processData: false
}).done(function(data, status, formXHR) {
  x = formXHR['responseText'].toString();
  var uploadedUrl = x.match("<Location>(.*)</Location>")[1];
  if ($(this).attr('id').startsWith('inp')) {
    if ($(this).attr('id').startsWith('inp')) $('footer').css('background-image', 'url(' + uploadedUrl + ')');
    footerBackground = $('footer').css('background');
  }
}).fail(function() {
  console.log("in ajax fail");
}.bind(this));

imgPromises.push(imgPromise1);

imgPromise2 = $.ajax({
  url: s3Url,
  type: "POST",
  data: s3FormData,
  mimeType: "multipart/form-data",
  contentType: false,
  cache: false,
  processData: false
}).done(function(data, status, formXHR) {
  x = formXHR['responseText'].toString();
  var uploadedUrl = x.match("<Location>(.*)</Location>")[1];
  if ($(this).attr('id').startsWith('inp')) {
    if ($(this).attr('id').startsWith('inp')) $('footer').css('background-image', 'url(' + uploadedUrl + ')');
    footerBackground = $('footer').css('background');
  }
}).fail(function() {
  console.log("in ajax fail");
}.bind(this));

imgPromises.push(imgPromise2);

Promise.all(imgPromises.then(function() {});

If any of the promise(imgPromise1 or imgPromise2) fails then it didn't go to Promise.all.

I want that in every case it should go to Promise.all.

Answer 1

Depending on the version of jQuery you are using you can use .catch and return something in catch that the promise will resolve with.

jQuery's version of Promise.all ($.when) will not reject if any of the deferred (is jQuery promise implementation) rejects.

This is only since version 3 since jQuery's deferred before this version did not behave like standardized promises (the promises native to modern browsers).

function makeRequest(num){//returning $.Deferred like $.ajax will
  var d = $.Deferred();
  setTimeout(() => {
	if(num>1){//reject if number passed is higher than 1
      d.reject("Rejecting over one");
      return;
    }
  d.resolve(num)
  }, 10);
  return d.promise();
}

$.when.apply(//jQueries Promise.all need apply to pass array of "promises"
  $,
  [1,2].map(//map numbers 1 and 2 to deferred 1 will resolve 2 rejects
    function(num){
      return makeRequest(num)
      .catch(//catch rejected deferred
        function(error){return "error in:"+num; }//resolve with this
      );
    }
  )
).then(//both deferred (=jQuery promise like) are resolved
  function(){
    console.log("it is done",[].slice.apply(arguments));
  }
)

<script
  src="https://code.jquery.com/jquery-3.3.1.min.js"
  integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
  crossorigin="anonymous"></script>

However, you try the same with version 2 of jQuery and it won't work because implementation of deferred/promise is not like standard promises in versions lower than 3. You could convert Deferred to real promise but if you want to support older browsers you need Promise polyfil:

console.log("jQuery version",jQuery.fn.jquery);

function makeRequest(num){//function returning jQuery deferred (could be $.ajax)
  var d = $.Deferred();
  if(num>1){
    //now this will throw an error
    throw("Rejecting over one");
  }
  setTimeout(() => {
    d.resolve(num)
  }, 10);
  return d.promise();
}

Promise.all(
  [1,2].map(
    function(num){
      //convert deferred to real Promise (having catch) if makeRequest throws then promise is rejected
      return Promise.resolve().then(()=>makeRequest(num))
      .catch(
        function(error){return "error in:"+num; }//resolve to error in... string
      );
    }
  )
).then(
  function(){
    console.log("it is done",[].slice.apply(arguments));
  }
)

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Answer 2

you are using then in the wrong place.

const Fail = function(error){this.error=error;};//special Fail type value
const isFail = o=>(o&&o.constructor)===Fail;//see if item passed is fail type
const isNotFail = o => !isFail(o);//see if item passed is not fail type
Promise.all(imgPromises
  .map(
    p=>Promise.resolve(p)/**convert to real promise*/
  ).map(
    p=>p.catch(error=>new Fail(error))//single request failed, resolve with Fail value
  )
)
.then(function (responses) {
  // successes = responses.filter(isNotFail)
  // failed = responses.filter(isFail)
})
.catch(function (err) {
  //handle error
});

MDN page for Promise.all

Answer 3

To wait until all promises finish their tasks (resolve and/or reject) we return value of catch() for each promise.

In this case we use .then() to receive all information.

We can filter both rejected and resolved data using filterInformation helper function.

Example:

const mockResolvePromise = (message) => {
  return new Promise((resolve) => {
    resolve(message)
  })
}
  
const mockRejectPromise = (messageError) => {
  return new Promise((_, reject) => {
    reject(messageError)
  })
}

const ajax0 = mockResolvePromise({ nice: 'data' })
const ajax1 = mockRejectPromise('bad error');
const ajax2 = mockRejectPromise('semi bad error');
const ajax3 = mockRejectPromise('super bad error');
const ajax4 = mockResolvePromise({ hello: 'handsome' })

const promises = [ajax0, ajax1, ajax2, ajax3, ajax4];
// Now lets add catch
const promisesWithCatch = promises.map(p => p.catch(e => { return { error: e } }))
  
  
const filterInformation = (stuff) => {
  return stuff.reduce((prev, current) => {
    
    if (current.error) return { 
      ...prev, 
      errors: [...prev.errors, current.error]
    };
    
    return { ...prev, data: [...prev.data, current] }
  
  }, { errors: [], data: [] })
}
  
  Promise.all(promisesWithCatch)
    .then(filterInformation)
    .then(data => console.log(data))

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>