Node.JS behaves strange

Question

I have a variable called uids

var uids = [];

Then I write some value to it property

uids[16778923] = "3fd6335d-b0e4-4d77-b304-d30c651ed509"

But before it

if (!uids[user.id]) {
 uids[user.id] = generateKey(user);
}

This thing behaves ok. If I try to get the value of it property

uids[currentUser.id]

It will give me a value of this property. If I try to call some methods like

Object.keys(uids);

It will give me, what I expected. And here the mystery comes...

uids;

RAM rest in piece. See the node eating ram

I am very confused now. What's wrong?

Answer 1

This is because you are creating a huge array and node will reserve memory for it - who knows what comes. I'd say that's a scenario where you would use a Map (or a plain object, but Map feels better here.

var uids = new Map();
var key = 456464564564654;
if (! uids.has(key)) {
    uids.set(key, generateKey(user))
}

Answer 2

You are creating an empty array (length is zero), then you assign some value to an arbitrary index. This will make the array grow as big as the index and assign the value to that index. Look at this example using node.js REPL:

> var a = []
undefined
> a[5] = "something"
'something'
> a
[ , , , , , 'something' ]
> a.length
6

Instead of creating an array, you could create a Map() or an common javascript object (singleton). Javascript objects behave like Maps but only Strings can be used as keys. If you assign a Number to be key, javascript will convert it to String automatically.

Personally, I would go with objects because they perform better. Instantiating an object takes longer than instantiating a Map (and it doesn't seem like you need to create several groups of "uids"), but once done, adding new keys and retrieving values from any key in faster when using common objects. At least that's how things go in my node.js v6.7.0 on ubuntu 14.04 but you could try for yourself. And it would also make the least alteration to your code.

var uids = {} // common/ordinary empty javascript object instead of array.
if (!uids[user.id]) { // getting value from one key works the same.
    uids[user.id] = generateKey(user) // assignment works the same.
}
////
uids[16778923] = "3fd6335d-b0e4-4d77-b304-d30c651ed509" // key will be "16778923".
uids[16778923] // getting value for key "16778923" can be done using 16778923 instead of "16778923".
////
uids[currentUser.id] // still returning values like this.
Object.keys(uids) // still returning an array of keys like this. but they are all Strings.