Dr. McKay

SteamCommunity and TradeOfferManager need only cookies to operate. That's how they keep track of their session. SteamUser operates directly through Steam's CM servers, not through the web. It doesn't need cookies for anything; it's merely capable of creating them (via the CM). Thus, you can create cookies through either SteamUser or SteamCommunity, and then provide them to SteamCommunity and TradeOfferManager.
 
If you're already logging in with SteamUser then cookies are created automatically, even if you don't request them. Thus, it's unnecessary (and potentially problematic) to also create them via SteamCommunity.
 
Basically, you want:
user.on('webSession', function(sessionID, cookies) { manager.setCookies(cookies, function(err) { if (err) { throw err; } }); community.setCookies(cookies); }); community.on('sessionExpired', function() { user.webLogOn(); }); It's also potentially helpful to just automatically call webLogOn every 30-60 minutes, just to be sure that it's always fresh. And finally, don't run the same account in two places or else the cookies have a nasty habit of expiring prematurely.

Listen for the sentOfferChanged event and see if the new state is Accepted.

Is steamcommunity up to date?

Yes, it's rate-limited by Steam. I can't tell you what the limit is, exactly, as I don't work for Valve.

No. If the error event is emitted, it's always fatal and it will never retry on its own.

promptSteamGuardCode is an option that must be set in the constructor, not in the logOn call.

loadUserInventory is deprecated. Use getUserInventoryContents instead.

Fetch the inventory for only tradable items, then call addTheirItems with the raw array of items. I believe there is an example in the repo that does exactly this.

The numbers which precede the market_hash_name are indeed the item's corresponding appid. All trading cards have that. You can also use the market_fee_app property.

You could if you had some kind of delimiter, but I don't know why you'd want to do that. You can't just naively assign the same data to multiple bots' pollData property.

You'd need to retrieve the TradeOffer object using getOffer or getOffers then call cancel() on that object.

The npm name is just steamcommunity.

You may.

You'd need to use node-steam-user for that, node-steamcommunity wouldn't work. Look around for the invite-related events.

5 seconds should be reasonably safe. 10 seconds is definitely safe.

https://steamerrors.com/16
 
No way to fix Steam timeouts. The only way to make the event faster is to lower your poll interval.

There is no way to represent a SteamID as a number in JavaScript, period. If you want to validate an individual SteamID from input, you can use regex: str.match(/^76561\d{12}$/)

You can only cancel an offer once. There's no limit on how many offers you can cancel on your account, though. If you send and cancel a million offers they might take notice.

That's the correct way to do it. Make sure you aren't trying to do any authenticated stuff before the webSession event gets emitted.

Currently, profile showcases aren't really supported.

manager.offer doesn't exist. You want this:
manager.on('receivedOfferChanged', (offer) => { community.postUserComment(offer.partner.toString(), 'Test'); });

It's happening in getUserInventoryContents. That means that the Steam inventory servers are busy and you'll need to try again later.

Yes, use the WebAPI: https://lab.xpaw.me/steam_api_documentation.html#ISteamUser_GetFriendList_v1

This is what the amount property is for.

Yes, it's caused by different device IDs.