Mongodb return array of strings instead array of objects

You don't need to do anything special. Just change one thing in your code:

Your code

UserMdl.find().exists('token').select('token').exec(function onUsersFound(err, userMdls) {
    console.log(userMdls);    // (*) Change this to...
});

Change

UserMdl.find().exec((err, users) => {
    console.log(users.map(user => user.token));    // ...(*) to this
});

No need to use select & exists functions

---

EDIT

The best option is Mongoose distinct function:

const tokens = await UserMdl.distinct('token');

If you want condition then:

const tokens = await UserMdl.find(/* condition */).distinct('token');

Thats it

There is a way to select only tokens that require less processing using the mongodb aggregation framework

UserMdl.aggregate(
  { $match: { token : { $exists : true }}},
  { $project: { _id: 0, token: 1 }},
  function onUsersFound(err, tokens) {
    console.log(tokens);
  });
);

This constructs an aggregation pipeline that first matches all documents that have a token field and then selects the token field and suppresses _id selection by using _id : 0 in the $project pipeline step.

The post processing step would look like this:

function postProcess(tokenObjects) {
  if (!tokenObjects) {
    return [];
  }

  return tokenObjects.map(function(tokenObject) { return tokenObject.token; });  
}

See also the mongoose docs for more detail on the aggregation function.