Q.nfcall or Q.denodeify for constructors

Question

I'm using a library providing a class whose constructor makes use of the Node.js callback pattern:

new FooBar({key: value}, function(err, data) {
  console.log(data);
});

I want to use a promise instead so that I can do something like this:

fooBarWrapper({key: value}).then(function(data) {
  console.log(data);
});

I know I can create a wrapper with Q.denodeify or call a function with Q.nfcall, but how does it work for constructors?

Constructors cannot return objects asynchronously. So, the constructor might be storing the callback function passed in it and invokes it whenever a particular event occurs. — thefourtheye, Dec 28 '14 at 14:52
Hard to answer without knowing your usage case for the constructor. — Roamer-1888, Dec 28 '14 at 15:01
I actually don't want to use the constructed object. I just need to call the constructor like any other static function. — Michel Krämer, Dec 28 '14 at 15:22
Then you probably don't need a constructor, just a regular function called without `new`. — Roamer-1888, Dec 28 '14 at 16:20
The library that I use does not provide another entry point. It's node-exif. — Michel Krämer, Dec 28 '14 at 16:30
related: [Is it bad practice to have a constructor function return a Promise?](http://stackoverflow.com/q/24398699/1048572) — Bergi, Dec 28 '14 at 22:06

score 1 · Answer 1 · answered Dec 28 '14 at 19:05

I managed to solve it myself. You can use Q.defer to convert the callback to a promise:

var deferred = Q.defer();
new FooBar({key: value}, deferred.makeNodeResolver());
deferred.then(function(data) {
  console.log(data);
});

I'm leaving this answer here in the hope that it helps anybody else having the same problem.

score 1 · Accepted Answer · answered Dec 28 '14 at 19:07

1

Constructors shouldn't make asynchronous calls. This is not fun to do because the person making this library are performing async IO in constructors which is totally not cool of them :)

I suggest you wrap that function in a callback:

function fooBar(val, cb){
    new FooBar(val, cb);
}

Which would let you call Q.nfcall or Q.denodeify from your code. I believe this is better than creating an explicit deferred.

answered Dec 28 '14 at 19:07

Benjamin Gruenbaum

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I totally agree. It's just not cool! :-) Thanks for the idea. – Michel Krämer Dec 29 '14 at 08:17

Q.nfcall or Q.denodeify for constructors

2 Answers2