How to get the size of a JavaScript object?

Question

I want to know the size occupied by a JavaScript object.

Take the following function:

function Marks(){
  this.maxMarks = 100;
}

function Student(){
  this.firstName = "firstName";
  this.lastName = "lastName";
  this.marks = new Marks();
}

Now I instantiate the student:

var stud = new Student();

so that I can do stuff like

stud.firstName = "new Firstname";

alert(stud.firstName);

stud.marks.maxMarks = 200;

etc.

Now, the stud object will occupy some size in memory. It has some data and more objects.

How do I find out how much memory the stud object occupies? Something like a sizeof() in JavaScript? It would be really awesome if I could find it out in a single function call like sizeof(stud).

I’ve been searching the Internet for months—couldn’t find it (asked in a couple of forums—no replies).

Answer 1

This is a hacky method, but i tried it twice with different numbers and it seems to be consistent.

What you can do is to try and allocate a huge number of objects, like one or two million objects of the kind you want. Put the objects in an array to prevent the garbage collector from releasing them (note that this will add a slight memory overhead because of the array, but i hope this shouldn't matter and besides if you are going to worry about objects being in memory, you store them somewhere). Add an alert before and after the allocation and in each alert check how much memory the Firefox process is taking. Before you open the page with the test, make sure you have a fresh Firefox instance. Open the page, note the memory usage after the "before" alert is shown. Close the alert, wait for the memory to be allocated. Subtract the new memory from the older and divide it by the amount of allocations. Example:

function Marks()
{
  this.maxMarks = 100;
}

function Student()
{
  this.firstName = "firstName";
  this.lastName = "lastName";
  this.marks = new Marks();
}

var manyObjects = new Array();
alert('before');
for (var i=0; i<2000000; i++)
    manyObjects[i] = new Student();
alert('after');

I tried this in my computer and the process had 48352K of memory when the "before" alert was shown. After the allocation, Firefox had 440236K of memory. For 2million allocations, this is about 200 bytes for each object.

I tried it again with 1million allocations and the result was similar: 196 bytes per object (i suppose the extra data in 2mill was used for Array).

So, here is a hacky method that might help you. JavaScript doesn't provide a "sizeof" method for a reason: each JavaScript implementaion is different. In Google Chrome for example the same page uses about 66 bytes for each object (judging from the task manager at least).

Answer 2

Many thanks to everyone that has been working on code for this!

I just wanted to add that I've been looking for exactly the same thing, but in my case it's for managing a cache of processed objects to avoid having to re-parse and process objects from ajax calls that may or may not have been cached by the browser. This is especially useful for objects that require a lot of processing, usually anything that isn't in JSON format, but it can get very costly to keep these things cached in a large project or an app/extension that is left running for a long time.

Anyway, I use it for something something like:

var myCache = {
    cache: {},
    order: [],
    size: 0,
    maxSize: 2 * 1024 * 1024, // 2mb

    add: function(key, object) {
        // Otherwise add new object
        var size = this.getObjectSize(object);
        if (size > this.maxSize) return; // Can't store this object

        var total = this.size + size;

        // Check for existing entry, as replacing it will free up space
        if (typeof(this.cache[key]) !== 'undefined') {
            for (var i = 0; i < this.order.length; ++i) {
                var entry = this.order[i];
                if (entry.key === key) {
                    total -= entry.size;
                    this.order.splice(i, 1);
                    break;
                }
            }
        }

        while (total > this.maxSize) {
            var entry = this.order.shift();
            delete this.cache[entry.key];
            total -= entry.size;
        }

        this.cache[key] = object;
        this.order.push({ size: size, key: key });
        this.size = total;
    },

    get: function(key) {
        var value = this.cache[key];
        if (typeof(value) !== 'undefined') { // Return this key for longer
            for (var i = 0; i < this.order.length; ++i) {
                var entry = this.order[i];
                if (entry.key === key) {
                    this.order.splice(i, 1);
                    this.order.push(entry);
                    break;
                }
            }
        }
        return value;
    },

    getObjectSize: function(object) {
        // Code from above estimating functions
    },
};

It's a simplistic example and may have some errors, but it gives the idea, as you can use it to hold onto static objects (contents won't change) with some degree of intelligence. This can significantly cut down on any expensive processing requirements that the object had to be produced in the first place.

Answer 3

If you need to programatically check for aprox. size of objects you can also check this library http://code.stephenmorley.org/javascript/finding-the-memory-usage-of-objects/ that I have been able to use for objects size.

Otherwise I suggest to use the Chrome/Firefox Heap Profiler.

Answer 4

I know this is absolutely not the right way to do it, yet it've helped me a few times in the past to get the approx object file size:

Write your object/response to the console or a new tab, copy the results to a new notepad file, save it, and check the file size. The notepad file itself is just a few bytes, so you'll get a fairly accurate object file size.

Answer 5

This Javascript library sizeof.js does the same thing. Include it like this

<script type="text/javascript" src="sizeof.js"></script>

The sizeof function takes an object as a parameter and returns its approximate size in bytes. For example:

// define an object
var object =
    {
      'boolean' : true,
      'number'  : 1,
      'string'  : 'a',
      'array'   : [1, 2, 3]
    };

// determine the size of the object
var size = sizeof(object);

The sizeof function can handle objects that contain multiple references to other objects and recursive references.

Originally published here.

Answer 6

I just wrote this to solve a similar (ish) problem. It doesn't exactly do what you may be looking for, ie it doesn't take into account how the interpreter stores the object.

But, if you are using V8, it should give you a fairly ok approximation as the awesome prototyping and hidden classes lick up most of the overhead.

function roughSizeOfObject( object ) {

    var objectList = [];

    var recurse = function( value )
    {
        var bytes = 0;

        if ( typeof value === 'boolean' ) {
            bytes = 4;
        }
        else if ( typeof value === 'string' ) {
            bytes = value.length * 2;
        }
        else if ( typeof value === 'number' ) {
            bytes = 8;
        }
        else if
        (
            typeof value === 'object'
            && objectList.indexOf( value ) === -1
        )
        {
            objectList[ objectList.length ] = value;

            for( i in value ) {
                bytes+= 8; // an assumed existence overhead
                bytes+= recurse( value[i] )
            }
        }

        return bytes;
    }

    return recurse( object );
}