array.prototype.flat 
An ES2019 spec-compliant Array.prototype.flat 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.flat 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.flatUsage/Examples
var flat = require('array.prototype.flat');
var assert = require('assert');
var arr = [1, [2], [], 3, [[4]]];
assert.deepEqual(flat(arr, 1), [1, 2, 3, [4]]);var flat = require('array.prototype.flat');
var assert = require('assert');
/* when Array#flat is not present */
delete Array.prototype.flat;
var shimmedFlat = flat.shim();
assert.equal(shimmedFlat, flat.getPolyfill());
assert.deepEqual(arr.flat(), flat(arr));var flat = require('array.prototype.flat');
var assert = require('assert');
/* when Array#flat is present */
var shimmedIncludes = flat.shim();
var mapper = function (x) { return [x, 1]; };
assert.equal(shimmedIncludes, Array.prototype.flat);
assert.deepEqual(arr.flat(mapper), flat(arr, mapper));Tests
Simply clone the repo, npm install, and run npm test