DEV Community

How to Get an Object Length

Samantha Ming on April 15, 2019

Unlike arrays, it's always been tricky to get the object length. Well no more! Object.keys return an array of all the object's enumerable proper...

Read full post

Eugene Karataev • Apr 15 '19

Btw, Chrome displays not enumerable properties in the console. It just shows them slightly differently - not as bright as enumerable ones.

Samantha Ming • Apr 17 '19 • Edited

Wooo... that's interesting! good to know, thanks for sharing that! Let me add this my notes 👍

Sébastien Vercammen • Apr 16 '19 • Edited

Semantics: A JS object is a collection of properties, keys that map to their respective values, with keys that can only be strings or symbols. It doesn't have a length.

Similar to asking "What is the length of your car?" and giving the answer "The length of my car is 14, because it has 4 doors, a trunk, 6 windows, and 4 wheels".

(Simplified example.)

Using Object.keys().length or Object.getOwnPropertyNames().length gives the length of the list of enumerable keys (or all property names for the latter), not of an object.

Not judging though. I just enjoy being explicit/intentional with wording when I can. Good article and thanks for sharing. :)

Something you could add: Using an object as a key calls .toString(), which will make all object keys conflict on the key "[object Object]". ES6' Map works with objects but uses their memory address as keys, so an exact copy of an object will have a different key (no deep comparison, for efficiency).

Samantha Ming • Apr 17 '19 • Edited

Totally fair assessment! I guess I always just associated length to the size, since length is the property being called for array or strings. So naturally when I was thinking of getting the size of an object, I automatically default to referring it to length as well 😅 I guess that's why Lodash opted for size and not length. This is probably the best thing with sharing an article. It always opens up a conversation. I'm glad you shared your comment! Now, I learned something. Thank you for explaining with such clarity! 😊

Sébastien Vercammen • Apr 18 '19

You're absolutely right. Semantics, to define the meaning of things, is great to have conversations about if you find the right people.

Meaning is flexible, after all, until something is defined. Exploring this creates new understanding and perspectives, which works wonders to break out of black/white thinking.

Daniel Escoz • Apr 16 '19

Maybe length is the wrong term, but it's perfectly reasonable to define an object's size the way it's done in this post.

Samantha Ming • Apr 17 '19

Thanks for chiming in Daniel! Now I can see why Lodash chose size instead of length 😅

Daniel Escoz • Apr 17 '19

Notice also that JavaScript itself is choosing size for types like Set or Map: size is a more generic (and mathematical) name that suits more needs better, but nitpicking and pedantry aside, length is perfectly a understandable (and I keep using it for Sets and Maps until I remember that's wrong).

Sébastien Vercammen • Apr 18 '19

Clarifying meaning isn't pedantic.

Using size and capacity instead of length on data structures with dynamic lengths was done before JS.

There was probably a discussion somewhere, where they argued for/against both.

Python implements len() as a function with the magic method __len__ precisely to have one consistent way of retrieving an intuitive "length".

David Wickes • Apr 15 '19 • Edited

I enjoyed this article! "How to get an Object Length" really undersells it. Didn't know about the enumerable property - or the weird thing with symbols. Great piece of writing.

How would you use these features? I can understand enumerable being useful to get some level of 'privacy'/encapsulation into an JavaScript object. Are symbols with keys the same thing but more so? Are there any other uses?

Samantha Ming • Apr 17 '19

Thanks David for the kind feedback! In regards to symbols, from the readings I gathered, it seems to serve one main purpose: create property keys without risking name collisions (since symbols are always unqiue). I have not personally used symbols in my daily programming yet. if you know of any other use cases, please do share. Might be an interesting one to cover in a future post, have to dig into it a bit further 😊

Nicky Meuleman • Apr 15 '19 • Edited

Great writeup!

The enumerable and Symbols parts were especially excellent.

This made me play around with your example code a bit and I have a question.

const numbers = {one: '1️⃣'};

Object.defineProperty(
  numbers,
  Symbol.for('two'), {
    value: '2️⃣',   
    enumerable: false // default value
  },
);

const enumerableLength = Object.keys(numbers).length; // 1
const symbolLength = Object.getOwnPropertySymbols(numbers).length; // 1
const totalObjectLength = enumerableLength + symbolLength; // 2

How should I handle this case where I don't want the symbol to get counted?

edit:
Came up with this, other solutions?

const enumerableSymbolLength = Object.getOwnPropertySymbols(numbers).filter(key => numbers.propertyIsEnumerable(key)).length // 0
const totalObjectLength = enumerableLength + enumerableSymbolLength; // 1

Samantha Ming • Apr 17 '19

If you only want the enumerable properties without the symbols. The Object.keys method should just work, since it doesn't include that. If I misunderstood your questions, please follow up 🙂

Nicky Meuleman • Apr 17 '19 • Edited

All enumerable properties, including ones with Symbols as keys.
Excluding all non-enumerable properties.

an object with:
1 enumerable property with string as key,
2 non-enumerable properties with a string as key,
1 enumerable property with Symbol as key
1 non-enumerable properties with Symbol as key.

Should return 2 as total length. (1 for string key and 1 for Symbol key)

Eugene Karataev • Apr 15 '19

Wait.
At first you said that Object.keys don't return hidden properties.
In the end of the article you use this method to count a number of object's properties. I guess you should use Object.getOwnPropertyNames(animal).length instead of Object.keys(animal).length.

Samantha Ming • Apr 17 '19

Which part? Hmm...i don't think i see it 😲

Eugene Karataev • Apr 17 '19

Here's your code in the end of the article to count total amount of an object's properties.

const enumerableLength = Object.keys(animal).length;
const symbolLength = Object.getOwnPropertySymbols(animal).length;
const totalObjectLength = enumerableLength + symbolLength;
// 2 <-- 👍

Let's take a look at this object:

let animal = {head: 1};
Object.defineProperty(animal, 'legs', {value: 4});

This object has two properties, but your code above will return 1. So my thought was that it's better to replace Object.keys with Object.getOwnPropertyNames to return correct amount of an object's total properties.

Marko Kolombo • Apr 15 '19

Deeply covered, nice!

As a workaround, what about Object.values().length ?

Ran Lottem • Apr 16 '19

I think Object.values and Object.keys will have the same length, as they both consider only enumerable properties:

Docs for Object.values

Marko Kolombo • Apr 16 '19 • Edited

true story...

const object1 = {
  a: 'somestring',
  b: 42,
  c: false,
  [Symbol('ghost')]: '👻'
};

console.log(Object.values(object1).length); //>3

Ran Lottem • Apr 16 '19

I'm getting 3 for the length of both keys and values. The arrays are ["a", "b", "c"] and ["somestring", 42, false], respectively.

Samantha Ming • Apr 17 '19

Yup, that works too! Now that i think of it, i should have cover those examples too 😅 next time 😜

Ran Lottem • Apr 16 '19 • Edited

Great write-up!

I've used getOwnPropertyNames in the past, along with recursive calls on an object's prototype, to get all of the properties that object has and that it inherited.

Samantha Ming • Apr 17 '19

Yup that's great use case to use getOwnPropertyNames. I think the key is knowing what you want and picking the right tool for it. Thanks for sharing your use case! 👍