General-purpose bucket sort in JS

bucketSort.js

bucketSort = function(src, k2i, v2k) {
    if (src.length <= 1) return src;
    k2i = k2i || function(k, d){ return (''+k).charCodeAt(d) || 0 };
    v2k = v2k || function(v){ return v };
    /* first we bundle elements with same keys */
    var valOf = Object.create(null), concat = Array.prototype.concat;
    src.forEach(function(v){
        var k = v2k(v);
        if (valOf[k])   valOf[k].push(v);
        else            valOf[k] = [v];
    });
    return concat.apply([],
        /* then we sort its keys ... */
        (function inner(src, depth){
            var buckets = [];
            /* spread the items into buckets */
            src.forEach(function(v) {
                var i = k2i(v, depth, src);
                if (buckets[i]) buckets[i].push(v);
                else            buckets[i] = [v];
            });
            return concat.apply([], buckets
                .filter(function(v){ return !!v })  /* no empty buckets */
                .map(function(v) {
                    return v.length === 1
                        ?   v 
                        :   inner(v, depth+1) /* recurse if necessary */
                })
            );
         })(Object.keys(valOf), 0)
        /* … and retrieve the values */
        .map(function(k){ return valOf[k]})
    );
};