function sortBy (coll, iteratee, callback) {
        var _iteratee = wrapAsync(iteratee);
        return map$1(coll, (x, iterCb) => {
            _iteratee(x, (err, criteria) => {
                if (err) return iterCb(err);
                iterCb(err, {value: x, criteria});
            });
        }, (err, results) => {
            if (err) return callback(err);
            callback(null, results.sort(comparator).map(v => v.value));
        });

        function comparator(left, right) {
            var a = left.criteria, b = right.criteria;
            return a < b ? -1 : a > b ? 1 : 0;
        }
    }
Sorts a list by the results of running each `coll` value through an async
`iteratee`.

@name sortBy
@static
@memberOf module:Collections
@method
@category Collection
@param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
@param {AsyncFunction} iteratee - An async function to apply to each item in
`coll`.
The iteratee should complete with a value to use as the sort criteria as
its `result`.
Invoked with (item, callback).
@param {Function} callback - A callback which is called after all the
`iteratee` functions have finished, or an error occurs. Results is the items
from the original `coll` sorted by the values returned by the `iteratee`
calls. Invoked with (err, results).
@returns {Promise} a promise, if no callback passed
@example

// bigfile.txt is a file that is 251100 bytes in size
// mediumfile.txt is a file that is 11000 bytes in size
// smallfile.txt is a file that is 121 bytes in size

// asynchronous function that returns the file size in bytes
function getFileSizeInBytes(file, callback) {
    fs.stat(file, function(err, stat) {
        if (err) {
            return callback(err);
        }
        callback(null, stat.size);
    });
}

// Using callbacks
async.sortBy(['mediumfile.txt','smallfile.txt','bigfile.txt'], getFileSizeInBytes,
    function(err, results) {
        if (err) {
            console.log(err);
        } else {
            console.log(results);
            // results is now the original array of files sorted by
            // file size (ascending by default), e.g.
            // [ 'smallfile.txt', 'mediumfile.txt', 'bigfile.txt']
        }
    }
);

// By modifying the callback parameter the
// sorting order can be influenced:

// ascending order
async.sortBy(['mediumfile.txt','smallfile.txt','bigfile.txt'], function(file, callback) {
    getFileSizeInBytes(file, function(getFileSizeErr, fileSize) {
        if (getFileSizeErr) return callback(getFileSizeErr);
        callback(null, fileSize);
    });
}, function(err, results) {
        if (err) {
            console.log(err);
        } else {
            console.log(results);
            // results is now the original array of files sorted by
            // file size (ascending by default), e.g.
            // [ 'smallfile.txt', 'mediumfile.txt', 'bigfile.txt']
        }
    }
);

// descending order
async.sortBy(['bigfile.txt','mediumfile.txt','smallfile.txt'], function(file, callback) {
    getFileSizeInBytes(file, function(getFileSizeErr, fileSize) {
        if (getFileSizeErr) {
            return callback(getFileSizeErr);
        }
        callback(null, fileSize * -1);
    });
}, function(err, results) {
        if (err) {
            console.log(err);
        } else {
            console.log(results);
            // results is now the original array of files sorted by
            // file size (ascending by default), e.g.
            // [ 'bigfile.txt', 'mediumfile.txt', 'smallfile.txt']
        }
    }
);

// Error handling
async.sortBy(['mediumfile.txt','smallfile.txt','missingfile.txt'], getFileSizeInBytes,
    function(err, results) {
        if (err) {
            console.log(err);
            // [ Error: ENOENT: no such file or directory ]
        } else {
            console.log(results);
        }
    }
);

// Using Promises
async.sortBy(['mediumfile.txt','smallfile.txt','bigfile.txt'], getFileSizeInBytes)
.then( results => {
    console.log(results);
    // results is now the original array of files sorted by
    // file size (ascending by default), e.g.
    // [ 'smallfile.txt', 'mediumfile.txt', 'bigfile.txt']
}).catch( err => {
    console.log(err);
});

// Error handling
async.sortBy(['mediumfile.txt','smallfile.txt','missingfile.txt'], getFileSizeInBytes)
.then( results => {
    console.log(results);
}).catch( err => {
    console.log(err);
    // [ Error: ENOENT: no such file or directory ]
});

// Using async/await
(async () => {
    try {
        let results = await async.sortBy(['bigfile.txt','mediumfile.txt','smallfile.txt'], getFileSizeInBytes);
        console.log(results);
        // results is now the original array of files sorted by
        // file size (ascending by default), e.g.
        // [ 'smallfile.txt', 'mediumfile.txt', 'bigfile.txt']
    }
    catch (err) {
        console.log(err);
    }
})();

// Error handling
async () => {
    try {
        let results = await async.sortBy(['missingfile.txt','mediumfile.txt','smallfile.txt'], getFileSizeInBytes);
        console.log(results);
    }
    catch (err) {
        console.log(err);
        // [ Error: ENOENT: no such file or directory ]
    }
}