How to write a function that returns an error in javascript

Question

In mongoose.js after every query the functions returns (error, result). I would like to write function of my own that does the same.

I thought the answer was to use promises so I wrote this

  login: (user) ->
    q.Promise (resolve, reject, notify) ->
      console.log resolve
      if _.has(user, 'password')
        dashboard.users.find(user).exec().then (err, results) ->
          if err then reject("Error occurred with the database")
          if results then resolve(results) else reject("User not found")
      else
        reject("A password is required for login")

Auth.login(testuser).then (d) ->
    console.log d, "done"

but after research I found that this is an anti pattern and the code does not work (promise is never resolved/rejected). So now I am not sure what to do.

How do you know the promise is never resolved/rejected? I don't see that you're using the actual promise anywhere. Did you mean to return the promise from the `login()` method? — jfriend00, Nov 26 '14 at 06:35
Like I said, you have to return the promise from the `login()` method so the callers of `login()` can use it. — jfriend00, Nov 26 '14 at 06:38
my original code had this but it is still the same result you are saying I should have deferred.promise returned at the end right? — grasshopper, Nov 26 '14 at 06:51
Avoid the [`Promise` constructor antipattern](https://stackoverflow.com/q/23803743/1048572?What-is-the-promise-construction-antipattern-and-how-to-avoid-it)! — Bergi, Aug 21 '22 at 14:13

score 2 · Answer 1 · answered Nov 26 '14 at 10:09

To return errors from functions as promises, there are generally two ways to produce rejected promises:

The reject function of your Promise library
Throwing an exception in a then callback or returning a rejected promise from it will reject the resulting promise

This has the same effect as building a promise for an async function that calls its node-style callback with an error argument. Your code would be rewritten to

  login: (user) ->
    if _.has(user, 'password')
      q(dashboard.users.find(user).exec()).then (results) ->
        if results 
          results
        else
          throw new Error("User not found")
      , (err) ->
        throw new Error("Error occurred with the database")
    else
      q.reject("A password is required for login")

How to write a function that returns an error in javascript

1 Answers1