array.prototype.groupToMap 
An ESnext spec-compliant Array.prototype.groupToMap 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.groupToMap 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.grouptomapUsage/Examples
var groupToMap = require('array.prototype.grouptomap');
var assert = require('assert');
var arr = [0, 1, 2, 3, 4, 5];
var parity = function (x) { return x % 2 === 0 ? 'even' : 'odd'; };
var results = groupToMap(arr, function (x, i, a) {
assert.equal(x, arr[i]);
assert.equal(a, arr);
return parity(x);
});
assert.deepEqual(results, new Map([
['even', [0, 2, 4]],
['odd', [1, 3, 5]],
]));var groupToMap = require('array.prototype.grouptomap');
var assert = require('assert');
/* when Array#groupToMap is not present */
delete Array.prototype.groupToMap;
var shimmed = groupToMap.shim();
assert.equal(shimmed, groupToMap.getPolyfill());
assert.deepEqual(arr.groupToMap(parity), groupToMap(arr, parity));var groupToMap = require('array.prototype.grouptomap');
var assert = require('assert');
/* when Array#groupToMap is present */
var shimmed = groupToMap.shim();
assert.equal(shimmed, Array.prototype.groupToMap);
assert.deepEqual(arr.groupToMap(parity), groupToMap(arr, parity));Tests
Simply clone the repo, npm install, and run npm test