array.prototype.groupby 
An ESnext spec-compliant Array.prototype.groupBy shim/polyfill/replacement that works as far down as ES3.
This package implements the es-shim API interface. It works in an ES3-supported environment and complies with the proposed spec.
Because Array.prototype.groupBy depends on a receiver (the this value), the main export takes the array to operate on as the first argument.
Getting started
npm install --save array.prototype.groupbyUsage/Examples
var groupBy = require('array.prototype.groupby');
var assert = require('assert');
var arr = [0, 1, 2, 3, 4, 5];
var parity = function (x) { return x % 2 === 0 ? 'even' : 'odd'; };
var results = groupBy(arr, function (x, i, a) {
assert.equal(x, arr[i]);
assert.equal(a, arr);
return parity(x);
});
assert.deepEqual(results, {
__proto__: null,
even: [0, 2, 4],
odd: [1, 3, 5],
});var groupBy = require('array.prototype.groupby');
var assert = require('assert');
/* when Array#groupBy is not present */
delete Array.prototype.groupBy;
var shimmed = groupBy.shim();
assert.equal(shimmed, groupBy.getPolyfill());
assert.deepEqual(arr.groupBy(parity), groupBy(arr, parity));var groupBy = require('array.prototype.groupby');
var assert = require('assert');
/* when Array#groupBy is present */
var shimmed = groupBy.shim();
assert.equal(shimmed, Array.prototype.groupBy);
assert.deepEqual(arr.groupBy(parity), groupBy(arr, parity));Tests
Simply clone the repo, npm install, and run npm test