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