Is there a way, in Javascript (ES2015), to map only part of a array?

E.g.:

let m = someArray.map(function(n){ if(n===0) return n+1; }

That's it, it would return a mapped array that has less elements than the original one.

Is that something that we could accomplish out of the box with some functional programming technique?

Is there a way, in Javascript (ES2015), to map only part of a array?

E.g.:

let m = someArray.map(function(n){ if(n===0) return n+1; }

That's it, it would return a mapped array that has less elements than the original one.

Is that something that we could accomplish out of the box with some functional programming technique?

• javascript
• functional-programming
• ecmascript-6

• 4 How about a combination of .filter() and .map()? – Pointy Commented Sep 29, 2015 at 22:32
• 3 .reduce() could do that, if JS allows an array as initial value. Only needs 1 cycle then. – Rudie Commented Sep 29, 2015 at 22:33
• 2 .filter on n === 0 then .map – dievardump Commented Sep 29, 2015 at 22:37
• @Pointy never heard of .filter(). Any example? – Draconar Commented Sep 29, 2015 at 22:40
• 3 @Draconar it's easily google'able. Show some effort. – Bartek Banachewicz Commented Sep 29, 2015 at 22:41

4 Answers 4

Maps changing the number of elements are icky. It's better to filter first, then map:

let arr2 = arr.filter(e => e === 0).map(e => e + 1);

This is easy with array comprehensions, which were introduced by the ECMAScript 6 draft, but will be postponed to ES7:

let m = [for (n of someArray) if (n===0) n+1]

You won't be able to skip unneeded elements with native Array.prototype.map(). But there's always Array.prototype.reduce() to save the day:

let m = someArray.reduce(function(mem, el) {
  if (el === 0) {
    mem.push(el + 1);
  }
  return mem;
}, []);

Array iteration methods are (except for deprecated edge cases¹) a concise way of expressing what you'd do with a loop (for(i = 0; i < arr.length; i++) { doSomething(arr[i]) }). When the meaning of the method name is no longer representative, you acheive the exact opposite - instead of using a method that describe the operation, you're using one that doesn't and confuse the reader.

More so, not using this group of methods is more performant. This mostly happens because invoking a function is "expensive".

To use the methods, while being concise, see Bartek's answer.

Alternatively, simply use a loop:

let m = [];
for(v of someArray) {
  if(v !== 0) {
    m.push(v + 1);
  }
}

¹ When these methods are called, the length of the array is sampled, and any element added beyond this length from within the callback is not visited.