NodeJS waiting for recursive function results with mongoDB queries

Question

solution : not using forEach with a function which need to await for. using a simple for and adding await to the funcion.

I have a mongoose schema :

const TopicSchema = new mongoose.Schema({
    name: {type:String,unique:false},
    sub_topic:[{type:mongoose.Schema.Types.ObjectId, ref : 'Topic'}] 
});

each topic may have subtopics. I want to find for a given topic all of its sub and its sub-sub topic IDs. I wrote a recursive function to search deeper every time and retrive more subs.

Data now is : A-->
                  B-->no subs
                  C-->C1,C2
                  D-->D1,D2,D3

the recursive function:

  async function getSubs(ID) {
    let subs = [];
    const topic = await Topic.findById(ID);
    console.log('-------in topic', topic.name)
    if (topic.sub_topic.length == 0) {
      //stop condition-no more subs just return array with id
      return [ID];
    }
    topic.sub_topic.forEach(function (sub) {
      subs.push(sub._id); //add current sub id to array
      var data = getSubs(sub._id)
        .then((data) => {
          subs = subs.concat(getSubs(data));
        });
    })
    return subs; //finally return all found subs
  }

and this is how i invoke it :

app.get('/api/topic/subs/:id', (req, res, next) => {
let subs;
subs = getSubs(req.params.id)
  .then((subs) => {
    console.log('returned subs', subs)
  });

});

the problem is the function returns the subs but not waiting for all of the recursive calls to complete their job of adding more subs. this is the output :

-------in topic A
returned subs [ 5bcb558ba68fb623e4d97ae1,
  5bcb558ba68fb623e4d97ae4,
  5bcb558ca68fb623e4d97ae5 ]
-------in topic C
-------in topic B
-------in topic D
-------in topic C1
-------in topic C2
-------in topic D2
-------in topic D1
...
...and so on

how can i make it wait for the other calls to finish?

-Thank you!