Promise.All returning empty

Question

I have few api call requests which i am trying to create promises and then execute all using Promise.all but, its giving empty value instead of array. Below is my code.

function getUser(arrUser) {
  var student = [];
  return new Promise((resolve, reject) => {
    if (arrUser.length > 0) {
      var promises = arrUseridRequest.map(userRequeset => {
        return getRequest(userRequeset).then(result => {
          student.push(JSON.parse(result.body).result[0].Name);
          console.log(student); //This is giving right details.
        }).catch(error => {
          reject(error);
        });
      });

      Promise.all(promises).then(StuName => {
        resolve(StuName.join());
      })
    }
  });
}

and this is how i am trying to get the values at once:

getUser('123').then(student => {
  console.log(Student) //Coming as empty
});

getRequest is my api call nodeJs request module. What's wrong in my code?

You are using promises as a fancy callback system. Promise represents a value itself. So you need to create an array of promises returning something, not populating some external array. Then use Promise.all to produce final result. — Yury Tarabanko, Jul 15 '20 at 17:20
`console.log(Student)` Is this a typo? Shouldn’t this be `student` with small `s`? — Abhishek Sharma, Jul 15 '20 at 17:20
not sure you need to the student array to store the results or even the `new Promise` bit. I think... nm @YuryTarabanko just replied what I was going to get to. — akaphenom, Jul 15 '20 at 17:20
Where is `arrUser` parameter used? What is `arrUseridRequest`? — adiga, Jul 15 '20 at 17:20
Also you rarely need to use `Promise` constructor directly. Definitely not in the example you have. `Promise.all` creates one for you. — Yury Tarabanko, Jul 15 '20 at 17:21
many issues with the code sample. My recommendation is to limit what you are trying to do and functionality from a basis of it working... e.g. get it working for jsut one record, not an array. — akaphenom, Jul 15 '20 at 17:21
@YuryTarabanko Could you please help me how that can be done? — Lara, Jul 15 '20 at 17:23
`function getUsers(users) { return Promise.all( users.map(user => /* create a promise returning what you need for each user*/)}` can't write anything more useful because you have some random variables popping here and there in your code snippet. — Yury Tarabanko, Jul 15 '20 at 17:31
Avoid the [`Promise` constructor antipattern](https://stackoverflow.com/q/23803743/1048572?What-is-the-promise-construction-antipattern-and-how-to-avoid-it)! — Bergi, Jul 15 '20 at 17:39
`return getRequest(userRequeset).then(result => {` <--- this then returns nothing, you just push to an array. — epascarello, Jul 15 '20 at 17:45
@epascarello It still returns a promise from the `map` callback — Bergi, Jul 15 '20 at 17:49

Bergi · Answer 1 · 2020-07-15T18:01:10.967

All your promises fulfill with the value undefined since you're just logging the student names, but not returning them from the then callback. As you seem to be doing only a single request, the array will be [undefined], which is joined into the empty string.

Also avoid the Promise constructor antipattern:

function getUsers(arrUser) {
  const promises = arrUser.map(userId => {
    return getRequest(userId).then(result => {
      const students = JSON.parse(result.body).result;
      return students[0].Name;
    });
  });
  return Promise.all(promises);
}
getUsers(['123']).then(studentNames => {
  console.log(studentNames);
  console.log(studentNames.join());
}).catch(console.error);

Promise.All returning empty

1 Answers1