Array.prototype.find - Proposal for (breaking) change of API

Ron Buckton Ron.Buckton at
Tue Apr 7 20:56:35 UTC 2015

Even more off-topic, I’m sure, but with (a query library over iterables) and ES6 you can do:

import { Query } from './query';
let files = ["", ""];
let base = "/my/root";
let found = Query
  .map(file => path.resolve(base, file))
  .filter(file => fs.existsSync(file))


From: es-discuss [mailto:es-discuss-bounces at] On Behalf Of Andrea Giammarchi
Sent: Tuesday, April 7, 2015 1:40 PM
To: Alexander Jones
Cc: Martin Heidegger; es-discuss at
Subject: Re: Array.prototype.find - Proposal for (breaking) change of API

not sure if off topic but

  * you need a with statement to use lodash like that
  * ... the following ...

let found = first(filter(
  map(files, _ => path.resolve(base, _)),

is a very wasteful pattern ... have you ever heard of `some` ?

let found;
some(files, _ => fs.existsSync(found = path.resolve(base, _)) || (found = '') || false);

It could bring few alchemies in ;-)

On Tue, Apr 7, 2015 at 9:14 PM, Alexander Jones <alex at<mailto:alex at>> wrote:
You need lazy sequences aka iterators. They give you the rather optimal:

let found = first(filter(
    map(files, _ => path.resolve(base, _)),

Libs like lodash and lazy.js provide the bits you need, which are actually fairly trivial to build on top of nice new ES6 iterators if you want.


On Tuesday, April 7, 2015, Martin Heidegger <martin.heidegger at<mailto:martin.heidegger at>> wrote:
I am probably very late to the game but I just read about harmonies “Array.prototype.find” and found that it might be worth considering a change in way it works. Its not a drastic change but it would break the compatibility to the current setup. I understand that the chances for this going through will be slim but since I would like to give it a shot anyways:

Currently the API uses a callback function that has to return a boolean to identify the item that has been found. This is good because find & findIndex can use the same methods but it also has a drawback.

Problem: Say you have a list of items that need to be processed before you know if you “found” the item. Then you will need to process the last item again since there is no way to “keep” the result (A) of the find method or you preprocess all items using map (B) that will require to process all the items even if the first one already matches.


var files = [“<>”, “<>”]
var base = “/my/root"

// Specifies `path.resolve` on two places
var found = files.find(function (file) {
   return fs.existsSync(path.resolve(base, file))
if (found)
  found = path.resolve(base, file)

// Runs `path.resolve` for all items
var found = (file) {
  return path.resolve(base, file)

Proposal (C): I think it might be interesting to have a change of the signature so that the return value is not true/false but the value that actually will be returned (not undefined):

var found = files.find(function (file) {
   file = path.resolve(base, file)
     return file

This way the operations would be minimised, it is still few to write and it would make life a bit easier.

Martin Heidegger
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