flatMap-polyfill.js

if (!Array.prototype.flat) {
	Object.defineProperty(Array.prototype, 'flat', {
		configurable: true,
		value: function flat () {
			var depth = isNaN(arguments[0]) ? 1 : Number(arguments[0]);

			return depth ? Array.prototype.reduce.call(this, function (acc, cur) {
				if (Array.isArray(cur)) {
					acc.push.apply(acc, flat.call(cur, depth - 1));
				} else {
					acc.push(cur);
				}

				return acc;
			}, []) : Array.prototype.slice.call(this);
		},
		writable: true
	});
}

if (!Array.prototype.flatMap) {
	Object.defineProperty(Array.prototype, 'flatMap', {
		configurable: true,
		value: function flatMap (callback) {
			return Array.prototype.map.apply(this, arguments).flat();
		},
		writable: true
	});
}