JS Map from Object.entries memory usage

Question

I'm trying to find out the memory usage of javascript Map objects that have ~20k entries - key is a string, and value is an array of 1-2 strings. I have created 2 Maps: with 17k and 22k entries. They both have the same memory usage in chrome profiler - how?

Why do Map objects differ in size only after Objects from which they are created get removed from the scope? Also, i know how hashmaps work, but If someone knows how the js Map can perserve order please let me know (maybe just a linked list?).

function createMap(){
    var obj = JSON.parse(bigStringRepresentingTheJSON);
    return new Map(Object.entries(obj));
}

Looking at the profiler I see that both Maps take 917kb - how can that be? But Object (obj) from which they were made takes 786kb and 1 572kb - which is reasonable.

So I thought maybe Map holds a pointer to obj from which it was created, and thats why they don't differ in size? Then, I used the createMap function so that obj gets garbage collected. Only then, Map objects take 1 9kb and 2.3kb, which is to be expected.

Jonas Wilms · Accepted Answer · 2019-07-30 10:33:27Z

1

Scaling a Map is complicated, also a Hashmap is more efficient if data size << map size, as then hashes collide less often. Therefore it makes sense that a Map allocates more than needed, it probably allocates a fixed size hashtable, then scales if needed.

If thats done and how large the Map is depends entirely on the implementation though.

So I thought maybe Map holds a pointer to obj from which it was created, and thats why they don't differ in size?

No, the Map only holds references to the values in obj.

edited Jul 30, 2019 at 10:33

answered Jul 30, 2019 at 10:24

Jonas Wilms

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3 Comments

Nikola Dim Over a year ago

Thanks! You said "no" on my statement, then said the same thing, except "reference" instead of "poiter". Why does the Map memory usage then increase the moment when obj gets out of scope?

Jonas Wilms Over a year ago

I do have the feeling you draw the wrong conclusions from the metrics you see. Also please read carefuly. "the values of obj" does not mean obj.

Nikola Dim Over a year ago

Yea, it seemed wrong to me so I asked the question. Dunno why would obj getting out of scope make profiler show map taking up more memory.

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