The idea behind this concept is to more closely resemble ECMA 7 async functions by annotating functions. If your implementation returns a promise or uses a generator as a coroutine, it's an async function and this tries to make that more clear.

async.js

'use strict';

var Promise = require('bluebird');

var generatorSample = function *()
{
};

function isGenerator(func) {
    return func instanceof generatorSample.constructor;
}
/**
 * ECMA 7 async annotation
 * @param func
 * @returns {*}
 */
function async(func) {
    if (isGenerator(func)) {
        return Promise.coroutine(func);
    } else {
        return Promise.method(func);
    }
}

example.js

'use strict';

var async = require('async');

var somethingAsync = async(function *(){
    yield fooAsync();
    yield barAsync();
});

var somethingElseAsync = async(function (){
    if (someCondition()) {
        return barAsync();
    } else {
        return null;
    }
});