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First of all, you have to add that field in your schema in class object,

You need to join all three collections,

• $unwind to deconstruct the array
• $lookup there are two kind of lookups which help to join collections. First I used Multiple-join-conditions-with--lookup, and I used standrad lookup to join Users and Teams collections.
• $match to match the user's id
• $expr - when you use $match inside lookup, u must use it.
• $set to add new fields
• $group to we already destructed using $unwind. No we need to restructure it

here is the code

Another guess, adding to my comments: I just noticed that both version A and version B seems to return the final values as read from the dummies' properties.

If you have measured the time of version A and version B exactly as they are shown here, then do note that you're measuring not only the time of accessing the data via one or the other way, but you are also measuring the time for setting up all things.

For example, in version A, you are probably saving some time using y.GetType().GetProperties() which gets all props in one go, while in version B you're doing something probably very wasteful by going over some "properties" list and separately looking up each property by name: var prop = typeof(DummyType).GetProperty(propName);

Furthermore, if you measured it as shown, then version B includes generating dynamic assembly (or assembles) at runtime at the point where you .Compile(); the expression. That might have costed a lot of time, that could add a lot to the statistics.

So... I think you have to seriously revise your question. It's missing a lot of important information, and all we can do is guessing.

Map was added to the ECMAScript standard library in ECMAScript 2015. This is not just "something like a map", this is a map.

Here is a question with an answer of mine that uses a Map: How to declare Hash.new(0) with 0 default value for counting objects in JavaScript?

You can use jest.spyOn(object, methodName) to mock firebase.firestore() function.

E.g. index.jsx: