How promisifyAll works, or what are the requirements for it work?

Question

In a promise library bluebird have function promisifyAll or other similar libraries that claim to convert async functions with callback patterns into promise based ie. resolve(), reject(), or done()..So how does it work?

For example:

function myAsync1 (data, url, callBack) {...}

and if i put it in

Promise.promisify(myAsycn1);

then will my function work like this..

myAsync1('{..}', 'http://..').then(function(){...});

This is have been bothering me. Is there a pattern that async non promise libs or function need to follow for Bluebird promisifyAll to convert them to promises based methods or there is some magic that converts them.

If not then what are the requirements and how does it work with existing libraries like mongodb etc.

Answer 1

Is there a pattern that async non promise libs or function need to follow for Bluebird promisifyAll to convert them to promises based methods

Yes, there is a pattern. The functions it converts must expect a callback as their last argument. Additionally, it must pass an error as the first argument to the callback (null if no error) and the return value as the second argument.

The BlueBird promisify function is very difficult to follow because of optimizations, so I'll show a simple way it could be written:

function promisify(fn) {
  return function() {
    var that = this; // save context
    var args = slice.call(arguments); // turn into real array
    return new Promise(function(resolve, reject) {
      var callback = function(err, ret) { // here we assume the arguments to
                                          // the callback follow node.js
                                          // conventions
        if(err != undefined) {
          reject(err);
        } else {
          resolve(ret);
        }
      };
      fn.apply(that, args.concat([callback])); // Now assume that the last argument will
                                              // be used as a callback
    });
  };
}

Now we could implement promisifyAll by looping over the functions in the target object and using promisify on each one.