Is there is an easy and fast way to delete x keys from javascript object without looping over keys?

Question

I have a huge javascript object returned to a GET api call and i want to offer option to limit the number of keys returned like this:

GET /api/
{
   "bla1" : {
     ....
   },
   "bla2" : {
     ....
   },
   "bla3" : {
     ....
   },
   "bla4" : {
     ....
   },
   "bla5" : {
     ....
   },
   "bla6" : {
     ....
   },
   "bla7" : {
     ....
   }
}

when limit=2 is added to query params, this should be returned:

GET /api?limit=2
{
   "bla1" : {
     ....
   },
   "bla2" : {
     ....
   }
}

my main concern is response time. what if i have 100000 objects, i really don't want to loop over them. what shall i do?

EDIT: okey it doesn't have to be last. someone said it is arbitrary. so i guess it could be removed from any place.

@Duaa Zahi, If it is ordered data, then I think you should not use `Objects`. Are you keys numeric? Do you want this in client or server side? When you say `limit=2`, that seems to be a server side solution. — Nishant, Apr 03 '19 at 08:26
@Flimzy technically... they are ordered, though I share your views and opinions, they are not 100% accurate. We should not really on the ordering though, so I wholeheartedly agree with the comment — Dellirium, Apr 03 '19 at 08:27
This seems like a query string.. Are you trying to do that server-side? — wentjun, Apr 03 '19 at 08:28
Have you considered forming an array using the keys of the object? — Jack Bashford, Apr 03 '19 at 08:30
@Nishant they are not numeric. i just put this as example. this is sever side api. also i don't care about order, i just want the last x properties to be removed from the returned object — Duaa Zahi, Apr 03 '19 at 08:30
@DuaaZahi how can it be "last" if you do not care about the order? — Dellirium, Apr 03 '19 at 08:32
@Flimzy it is not arbitrary, there is a set of rules: http://www.ecma-international.org/ecma-262/6.0/#sec-ordinary-object-internal-methods-and-internal-slots-ownpropertykeys but I agree with you, just want to spread the information. I myself was not aware of this until recently — Dellirium, Apr 03 '19 at 08:35
Wow nice XY problem here. Where is this object created in the first place? That's where you should implement this filter. — Kaiido, Apr 03 '19 at 08:39
@Dellirium: This has changed, then. A JS object used to be defined as "an unordered collection of properties". See [here](https://stackoverflow.com/a/5525820/13860). I see as of ECMAScript 9, this is simply "A collection of properties". I'd say, relying on order is still fool-hardy. — Jonathan Hall, Apr 03 '19 at 08:40
@Flimzy wholeheartedly agree. Just want to keep everyone informed, as this was brought up to me as well where I was still thinking about them like you did. — Dellirium, Apr 03 '19 at 08:42

Jonathan Hall · Accepted Answer · 2019-04-03T08:56:12.327

There's really no way to accomplish your goal, without looping over at least part of your object. But you can limit your looping, by only looping over part of the object: Either the part you want to keep, or the part you want to discard.

It's easy to fetch N elements from any object, looping over only the keys to be kept:

let c = 0;
let target = {};
for (var property in object) {
  if (object.hasOwnProperty(property)) {
    target[property] = object[property];
    c++;
    if (c > N) {
        break;
    }
  }
}

You could also take the reverse approach, and delete N keys, looping over only the keys to be deleted:

let c = 0;
for (var property in object) {
  if (object.hasOwnProperty(property)) {
    delete object[property];
    c++;
    if (c > N) {
        break;
    }
  }
}

score 1 · Answer 2 · answered Apr 03 '19 at 09:10

1

How about:

 Object.entries(obj).slice(0, n).reduce(
     (acc, [k, v]) => (acc[k] = v, acc)
 );

Technically that's also a loop underneath, but over n elements selected upfront.

Mind you - the previous example is more memory efficient.

answered Apr 03 '19 at 09:10

Michał Karpacki

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Is there is an easy and fast way to delete x keys from javascript object without looping over keys?

2 Answers2