How can I convert a returned object to a promise with the object?

Question

I have an API call that returns either a promise or a object with information.

getFollowers(uid, auth){
    if(uid){
      var followersPromise =  this.db.collectionGroup(`following`)
      ....
      })
        .catch(function(error) {
          ...
      });
      return followersPromise
    }else{
      return {success: 0, error: "Item missing"}
    }
  }

In the case where a uid isn't supplied, I return a simple error message. However, if there is, it returns the promise. To make them consistent, I would like to return the object as a promise, even though it's not waiting on anything.

If this is a decent approach, how can I achieve it?

Answer 1

There are two approaches:

1. If the code initiating the operation can't even start it, it's reasonable to throw an error. That would be synchronous.

2. In situations where the asynchronous process is already underway and you're doing this, or if you simply prefer that the function never throws and always returns a promise, use Promise.reject:

   return Promise.reject(new Error("Item missing"));
   

(You don't have to use an Error with Promise.reject, just like you dont' have to with throw, but it's best practice in both cases.)

---

Two other comments on getFollowers:

1. In general, if you already have a promise to work with, e.g. from this.db.collectionGroup in this case, don't use new Promise. Just chain from that promise. See this question's answers for details.

2. I see in your question that getFollowers has a .catch handler. That's usually not appropriate if the function returns a promise. Just allow the error to propagate.

So if you want to do a synchronous error when you can't even initiate the operation:

getFollowers(uid, auth){
    if(!uid){
        throw new Error("Item missing");
    }
    return this.db.collectionGroup(`following`)
        // ....
    });
}

if you prefer to use a rejected promise instead:

getFollowers(uid, auth){
    return !uid
        ? Promise.reject(new Error("Item missing"))
        : this.db.collectionGroup(`following`)
            // ....
        });
}

Answer 2

Thats what Promise.resolve is for:

 return Promise.resolve({success: 0, error: "Item missing"});

Or you could just make the function async:

 async getFollowers(uid, auth){
   if(uid) {
     try {
      const result =  await this.db.collectionGroup(`following`);
      return result;
     } catch(error) {
       //  ...
     }
  } else {
    return {success: 0, error: "Item missing"}
  }
}