array.prototype.at 
An ESnext spec-compliant Array.prototype.at 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.at 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.atUsage/Examples
var at = require('array.prototype.at');
var assert = require('assert');
var arr = [1, [2], [], 3];
var results = at(arr, function (x, i) {
assert.equal(x, arr[i]);
return x;
});
assert.deepEqual(results, [1, 2, 3]);var at = require('array.prototype.at');
var assert = require('assert');
/* when Array#at is not present */
delete Array.prototype.at;
var shimmedFlatMap = at.shim();
var mapper = function (x) { return [x, 1]; };
assert.equal(shimmedFlatMap, at.getPolyfill());
assert.deepEqual(arr.at(mapper), at(arr, mapper));var at = require('array.prototype.at');
var assert = require('assert');
/* when Array#at is present */
var shimmedIncludes = at.shim();
var mapper = function (x) { return [x, 1]; };
assert.equal(shimmedIncludes, Array.prototype.at);
assert.deepEqual(arr.at(mapper), at(arr, mapper));Tests
Simply clone the repo, npm install, and run npm test