Mongoose - What does the exec function do?

Basically when using mongoose, documents can be retrieved using helpers. Every model method that accepts query conditions can be executed by means of a callback or the exec method.

callback:

User.findOne({ name: 'daniel' }, function (err, user) {
  //
});

exec:

User
  .findOne({ name: 'daniel' })
  .exec(function (err, user) {
      //
  });

Therefore when you don't pass a callback you can build a query and eventually execute it.

You can find additional info in the mongoose docs.

UPDATE

Something to note when using Promises in combination with Mongoose async operations is that Mongoose queries are not Promises. Queries do return a thenable, but if you need a real Promise you should use the exec method. More information can be found here.

During the update I noticed I didn't explicitly answer the question:

Ive never seen that method in Javascript before? What does it do exactly?

Well it's not a native JavaScript method, but part of the Mongoose API.

I never use exec() function to complete a CRUD(Create, Read, Update, Delete) on a model. When I want CRUD on a model, I use it like this:

const user = await UserModel.findOne(userCondition);

And it always does the job. So I wondered "what does exec() use for"? As I searched in mongoose document, I found the answer here.

Should You Use exec() With await?

And here is the story.
You have two ways to execute a query on a model. Using callback or using exec() function. "But" you can use await too. exec() function returns a promise, that you can use it with then() or async/await to execute a query on a model "asynchronous". So the question is "If I can just use user = await UserModel.find() and it works currectly, so why should I use exec() function?". The answer that you can find in the document is:

There are two difference between using await with exec() or without it.

• As a functionality point of view, there is no difference between using await with exec() or without it. Just when you call a query without exec() or callback, it returns a thenable which is something like promise but it's not a promise.(You can find the difference here). But when you use exec() to run a query, you get exactly a promise as response.

// returns a thenable as response that is not a promise, but you can use await and then() with it.
const user = await UserModel.findOne(userCondition);

// returns exactly a promise.
const user = await UserModel.findOne(userCondition).exec(); 

• Another difference is, if you use await with exec() you get a better "stack trace" if you catch any error in executing queries. So:
  
  These two line, do the same thing:

const user = await UserModel.findOne(userCondition);

// does exactly as the before line does, but you get a better stack trace if any error happened
const user = await UserModel.findOne(userCondition).exec(); 

exec() will return a promise if no callback is provided. So the following pattern is very convenient and generic - it can handle callbacks or promises nicely:

function findAll(query, populate, cb) {

  let q = Response.find(query);

  if (populate && populate.length > 0) {
    q = q.populate(populate);
  }

  // cb is optional, will return promise if cb == null
  return q.lean().exec(cb);

}

I recommend using Bluebird promises with Mongoose, to do that, use this call:

const mongoose = require('mongoose');
mongoose.Promise = require('bluebird');

Daniel has answered this quite beautifully. To elaborate on an exhaustive list of ways to build and execute queries, look at the following use cases:

Query Building

Mongoose will not execute a query until then or exec has been called upon it. This is very useful when building complex queries. Some examples can include using the populate and aggregate functions.

User.find({name: 'John'}) // Will not execute

Execution via callback

Although disliked by many due to its nesting nature, queries can be executed by providing the optional callback.

User.find({name: 'John'}, (err, res) => {}) // Will execute

Then API as a Promises/A+

Mongoose queries do provide a then function. This is not to be confused with regular promises. Simply put, the Promises/A+ specification requires a then function to work much like how we're used to with promises.

User.find({name: 'John'}).then(); // Will execute
Promise.all([User.find({name: 'John'}), User.find({name: 'Bob'})]) // Will execute all queries in parallel

The exec function

From Mongoose docs If you need a fully-fledged promise, use the .exec() function.

User.find({name: 'John'}).exec(); // Will execute returning a promise