Javascript flatMap implementation

flatMap.js

// [B](f: (A) ⇒ [B]): [B]  ; Although the types in the arrays aren't strict (:
Array.prototype.flatMap = function(lambda) { 
    return Array.prototype.concat.apply([], this.map(lambda)); 
};

function flatMap<T, U>(array: T[], callbackfn: (value: T, index: number, array: T[]) => U[]): U[] {
    return [].concat(...array.map(callbackfn));
}