AKA a “fake” promise vs a “real” promise — explained in the context of Mongoose Queries

I was happily doing some backend coding and at the part where I had to write a mongoose query to return a list of items, I got confused at whether I should chain .exec() or .then() behind the myModel.find() method. So I ended up searching their docs in an attempt to (try to) figure out which is the one I should use. Here’s a paragraph from their documentation:

Mongoose queries are not promises. They have a .then() function for co and async/await as a convenience. If you need a fully-fledged promise, use the .exec() function.

At that moment I was like…what’s a “fully-fledged promise”? What does it even mean in this context?

And the journey of trying to figure out what that phrase means got me to this link and I found out that what they meant was that Mongoose queries are thenables and not promises. According to that link,

“promise” is an object or function with a then method whose behavior conforms to this specification.

“thenable” is an object or function that defines a then method.

Okay so naturally, the question now is — what does “whose behavior conforms to this specification” mean? To answer this question, let us take a closer look at what a “real” A.K.A “fully-fledged promise” means (and then we’ll look at why it’s different from thenables after that).

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