flatMap function on Array.prototype

flatMap.ts

interface Array<T> {
	flatMap<TResult>(selector: (source: T) => Array<TResult>): Array<TResult>;	
}

Array.prototype.flatMap = function<T, TResult>(selector: (source: T) => Array<TResult>) {
	return (this as Array<T>).map(value => selector(value)).reduce((acc, value) => {
		return acc.concat(value);
	});
};