Fast alternative to Object.keys(obj).length

Question

How to get number of properties in an Object without the performance penalty that is associated with Object.keys().length?

Possible duplicate of Length of a JavaScript object (that is, associative array) — Oriol
– Oriol, Commented May 23, 2016 at 3:30
Possible duplicate of How to efficiently count the number of keys/properties of an object in JavaScript? — Paarth
– Paarth, Commented May 23, 2016 at 3:31
out of curiosity, what "performance penalty" are you speaking of? — dandavis
– dandavis, Commented May 23, 2016 at 3:33
@guest271314 How exactly do you use JSON.stringify() to count properties? — Oriol
– Oriol, Commented May 23, 2016 at 3:44

Joe Thomas · Accepted Answer · 2016-05-23 03:44:23Z

3

You could do a for-in loop, it's pretty fast. Basically it iterates through all the keys in the object and increments the counter for each one. Based on a quick benchmark in my console, it's about 5 times faster than Object.keys().length

var counter = 0;
for(var i in obj){
  counter++;
}

answered May 23, 2016 at 3:44

Joe Thomas

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Comments

Oriol · Accepted Answer · 2016-05-23 03:34:47Z

1

If you want a fast approach, basically you have two possibilities:

Keep track of the size of your object manually. Store the size in a variable, increase it when creating a property, and decrease it when deleting.
```
var myObj = {},
    size = 0;
myObj.foo = 1; ++size;
myObj.bar = 2; ++size;
size; // 2
```

Use a similar data structure which exposes the size. For example, ES6 maps.

var myMap = new Map();
myMap.set("foo", 1);
myMap.set("bar", 2);
myMap.size; // 2

answered May 23, 2016 at 3:34

Oriol

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11 Comments

dandavis Over a year ago

in that code, it seems using Map() isn't actually any faster than using a plain object and running Object.keys(). i realize it's a stub, but would you expect Map to be faster on a larger collection?

Oriol Over a year ago

@dandavis I don't only expect it. It must be faster. Map operations are required to be sublinear (on average) with the size of the map, Object.keys is linear (unless it has been cached or something).

dandavis Over a year ago

@Oriel: why do these result seemingly contradict what you say "must be faster": pagedemos.com/84xj3qxfm7cz/2 ? i ran them a bunch and i never saw a Map outperform an Object... i trust you (a lot), but i like to verify ;)

Oriol Over a year ago

@dandavis I think the test should be more like this. Map: 0.1, Obj: 2150. The advantage of maps is noticeable when the size is huge.

Patrick Evans Over a year ago

Object.keys should be taking longer as according to the spec size is a getter that loops the List entries and returning a count. Object.keys however does two loops: one in EnumerableOwnNames and then one in CreateArrayFromList of course speed also depends on the vendors implementation

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amaksr · Accepted Answer · 2016-05-23 03:52:21Z

0

You can switch to Map, which is similar to Object, but also you can get size of it, and iterate through keys. It is supported by many browsers, and for older ones there are polyfills.

edited May 23, 2016 at 3:52

answered May 23, 2016 at 3:33

amaksr

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1 Comment

Joe Thomas Over a year ago

While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes

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