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)); 
};

@ixth I think you must use apply() in order to convert the returned array of map() into the arguments of concat() :

Array.prototype.flatMap = function(lambda) { 
    return [].concat.appy([],this.map(lambda)); 
};

Without touching the prototype :

[0, 1, 2, 3, 4, 5].reduce((list,x) => list.concat([x, x+1]), []);

// [0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6]

Here is a bit of shorter way using es6 spread, similiar to renaudtertrais's - but using es6 and not adding to the prototype.

var flatMap = (a, cb) => [].concat(...a.map(cb))

const s = (v) => v.split(',')
const arr = ['cat,dog', 'fish,bird']

flatMap(arr, s)

To avoid adding an enumerable property (which will break all for (var i in []) {} statements, this adds the function as a non-enumerable property

Object.defineProperties(Array.prototype, {
    'flatMap': {
        value: function (lambda) {
            return Array.prototype.concat.apply([], this.map(lambda));
        },
        writeable: false,
        enumerable: false
    }
});

@AWilco +1. Note that writeable and enumerable both default to false, so you can leave those off.

Same implementation as @dsacramone with added TypeScript type annotations.
Note that mapping function can also use index and original array.

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

Not mentioned is that you should never modify prototypes of default objects, which this code perfectly failed to do.

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]