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)); 
};

If the stunt is performed by trained professionals that can be acceptable

For optimization, don't use concat use : push.apply

@Ran-P JSPerf begs to differ, for me at least https://jsperf.com/array-prototype-push-apply-vs-concat/13

@ichpuchtli I like it, but isn't the order of the concatenation backwards?

@dsacramone gets my vote for most concise and readable, tho :)

If you need to work with deeply nested arrays:

const myArray = [[1, 2],[3, [4, [5, 6]]], [7, [8, 9]]];

const flatMapDeep = (value, mapper) => {
  return Array.isArray(value) ?
    [].concat(...value.map(x => flatMapDeep(x, mapper))) :
    mapper(value);
}

const mapper = (x) => x * 11;
const flatArray = flatMapDeep(myArray, mapper); // [11, 22, 33, 44, 55, 66, 77, 88, 99]

const flatMap = (a, f) => a.map(f).reduce((xs, ys) => [...xs, ...ys]); // using map first to avoid recursion in reduce

[1, 2, 3].map(x => [x, x + 1]);                                        // => [[1, 2], [2, 3], [3, 4]]

flatMap([1, 2, 3], x => [x, x + 1]);                                   // => [1, 2, 2, 3, 3, 4]

/*
recursive methods (at least obvious ones) are for chumps! lets do some string manipulation instead...  
works, assuming your array does not actually includes "[" or "]" characters ¯\(◉◡◔)/¯
*/

Array.prototype.cheeky_flatMap = function(){
  return JSON.parse( "[" 
                   + JSON.stringify(this)
                         .replace(/[\[\]\,]+/g,",")
                         .replace(/(^\,|\,$)/g,"")
                   + "]"
                   );
}

Also, works in any depth...
cheeky_flatmap([[1,2],[3,4],[[[5]]]]) - [1,2,3,4,5]