How To Efficiently Count The Number Of Keys/properties Of An Object In Javascript

August 06, 2024 Post a Comment

What's the fastest way to count the number of keys/properties of an object? Is it possible to do this without iterating over the object? I.e., without doing: var count = 0; for (k

Solution 1:

To do this in any ES5-compatible environment, such as Node.js, Chrome, Internet Explorer 9+, Firefox 4+, or Safari 5+:

Object.keys(obj).length

Browser compatibility
Object.keys documentation (includes a method you can add to non-ES5 browsers)

Solution 2:

You could use this code:

if (!Object.keys) {
    Object.keys = function (obj) {
        var keys = [],
            k;
        for (k in obj) {
            if (Object.prototype.hasOwnProperty.call(obj, k)) {
                keys.push(k);
            }
        }
        return keys;
    };
}

Then you can use this in older browsers as well:

varlen = Object.keys(obj).length;

Solution 3:

If you are using Underscore.js you can use _.size (thanks douwe):

_.size(obj)

Alternatively you can also use _.keys which might be clearer for some:

_.keys(obj).length

I highly recommend Underscore.js. It's a tight library for doing lots of basic things. Whenever possible, they match ECMAScript 5 and defer to the native implementation.

Otherwise I support Avi Flax' answer. I edited it to add a link to the MDC documentation which includes the keys() method you can add to non-ECMAScript 5 browsers.

Solution 4:

The standard Object implementation (ES5.1 Object Internal Properties and Methods) does not require an Object to track its number of keys/properties, so there should be no standard way to determine the size of an Object without explicitly or implicitly iterating over its keys.

So here are the most commonly used alternatives:

1. ECMAScript's Object.keys()

Object.keys(obj).length; Works by internally iterating over the keys to compute a temporary array and returns its length.

Pros - Readable and clean syntax. No library or custom code required except a shim if native support is unavailable
Cons - Memory overhead due to the creation of the array.

2. Library-based solutions

Many library-based examples elsewhere in this topic are useful idioms in the context of their library. From a performance viewpoint, however, there is nothing to gain compared to a perfect no-library code since all those library methods actually encapsulate either a for-loop or ES5 Object.keys (native or shimmed).

3. Optimizing a for-loop

The slowest part of such a for-loop is generally the .hasOwnProperty() call, because of the function call overhead. So when I just want the number of entries of a JSON object, I just skip the .hasOwnProperty() call if I know that no code did nor will extend Object.prototype.

Otherwise, your code could be very slightly optimized by making k local (var k) and by using prefix-increment operator (++count) instead of postfix.

Baca Juga

var count = 0;
for (var k in myobj) if (myobj.hasOwnProperty(k)) ++count;

Another idea relies on caching the hasOwnProperty method:

var hasOwn = Object.prototype.hasOwnProperty;
var count = 0;
for (var k in myobj) if (hasOwn.call(myobj, k)) ++count;

Whether this is faster or not on a given environment is a question of benchmarking. Very limited performance gain can be expected anyway.

Solution 5:

Here are some performance tests for three methods;

https://jsperf.com/get-the-number-of-keys-in-an-object

Object.keys().length

20,735 operations per second

It is very simple and compatible and runs fast but expensive, because it creates a new array of keys, which then gets thrown away.

returnObject.keys(objectToRead).length;

Loop through the keys

15,734 operations per second

let size=0;
for(let k in objectToRead) {
  size++
}
return size;

It is slightly slower, but nowhere near the memory usage, so it is probably better if you're interested in optimising for mobile or other small machines.

Using Map instead of Object

953,839,338 operations per second

return mapToRead.size;

Basically, Map tracks its own size, so we're just returning a number field. It is far, far faster than any other method. If you have control of the object, convert them to maps instead.

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