Grouping in JavaScript

group-by.js

// A grouping function for JavaScript.
// Created by Ryan O'Hara.
// Usage:
//     group([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], function(x) { return x % 2; });
//     group({a: 20, b: 30, c: 40, d: 50}, function(x) { return x % 20; });
// Result:
//     [[1, 3, 5, 7, 9], [2, 4, 6, 8, 10]]
//     [{a: 20, c: 40}, {b: 30, d: 50}]

function group(obj, func, thisArg) {
    var resultKeys = [];
    var resultValues = [];
    var i, j, k, l;

    if(Object.prototype.toString.call(obj) === '[object Array]') {
        for(i = 0, l = obj.length; i < l; i++) {
            k = func.call(thisArg, obj[i], i, obj);
            j = resultKeys.indexOf(k);

            if(~j) {
                resultValues[j].push(obj[i]);
            } else {
                resultKeys.push(k);
                resultValues.push([obj[i]]);
            }
        }

        return resultValues;
    }

    for(i in obj) {
        if(obj.hasOwnProperty(i)) {
            k = func.call(thisArg, obj[i], i, obj);
            j = resultKeys.indexOf(k);

            if(~j) {
                resultValues[j][i] = obj[i];
            } else {
                resultKeys.push(k);
                resultValues.push({});
                resultValues[resultValues.length - 1][i] = obj[i];
            }
        }
    }

    return resultValues;
}