sortBy.js

import arrayMap from 'lodash/_arrayMap';
import property from 'lodash/_baseProperty';

import map from './map';

/**
 * 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|Object} coll - A collection to iterate over.
 * @param {Function} iteratee - A function to apply to each item in `coll`.
 * The iteratee is passed a `callback(err, sortValue)` which must be called once
 * it has completed with an error (which can be `null`) and a value to use as
 * the sort criteria. 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).
 * @example
 *
 * async.sortBy(['file1','file2','file3'], function(file, callback) {
 *     fs.stat(file, function(err, stats) {
 *         callback(err, stats.mtime);
 *     });
 * }, function(err, results) {
 *     // results is now the original array of files sorted by
 *     // modified date
 * });
 *
 * // By modifying the callback parameter the
 * // sorting order can be influenced:
 *
 * // ascending order
 * async.sortBy([1,9,3,5], function(x, callback) {
 *     callback(null, x);
 * }, function(err,result) {
 *     // result callback
 * });
 *
 * // descending order
 * async.sortBy([1,9,3,5], function(x, callback) {
 *     callback(null, x*-1);    //<- x*-1 instead of x, turns the order around
 * }, function(err,result) {
 *     // result callback
 * });
 */
export default function sortBy (coll, iteratee, callback) {
    map(coll, function (x, callback) {
        iteratee(x, function (err, criteria) {
            if (err) return callback(err);
            callback(null, {value: x, criteria: criteria});
        });
    }, function (err, results) {
        if (err) return callback(err);
        callback(null, arrayMap(results.sort(comparator), property('value')));
    });

    function comparator(left, right) {
        var a = left.criteria, b = right.criteria;
        return a < b ? -1 : a > b ? 1 : 0;
    }
}