Isn't it possible to concat new value to a field in MongoDB document while updating it?

I have few documents in a MongoDB Collection and each has a unique field called "requestid".
Now I need to update a field "requeststatus" by concatenating a new value to an existing one in NodeJS application. I started using MongoDB for very recent and have less exposure in it’s features.
After doing some research I got to know I could use "$set" with "$concat"

Updating with filter & options:

var filter = { requestid: data.requestid };
var updateDoc = { $set: { requeststatus: { $concat: ["$requeststatus","-",`. ${data.status}`] } } };
var options = { multi: true };
var jobDetails = { filter, updateDoc, options };

NodeJS code:

async function updateJobDetails(connection, data, mongoDetails){
const result = await connection.db(mongoDetails.mongoDatabase).collection(mongoDetails.collection).updateOne(data.filter, data.updateDoc, data.options);
}

This is not doing as expected, instead it’s adding the new concatenated value as array of Object into MongoDB collection.

Existing document:

{
"_id": {
    "$oid": "6307120d3oiu895oi9e82eea5"
},
"requestid": "123456789",
"iscancelled": true,
"organizationid": "3",
"instanceid": "172",
"offerid": "offer123",
"promotionid": "promo123",
"jobtype": "portaljob123",
"jobid": "job123",
"requeststatus": "began"
}

Updated document:

{
"_id": {
    "$oid": "6307120d3oiu895oi9e82eea5"
},
"requestid": "123456789",
"iscancelled": true,
"organizationid": "3",
"instanceid": "172",
"offerid": "offer123",
"promotionid": "promo123",
"jobtype": "portaljob123",
"jobid": "job123",
"requeststatus": {
    "$concat": ["$requeststatus", "-", ". tigger_datalink stopped since request was cancelled"]
}
}

Is there anything that I am doing wrong here? I even tried updateMany() but of no use. Run it as many times as desired it won’t concat but keep updating same value as Object
Any help is appreciated here.

>Solution :

In order to use an existing field’s data you need to use an update with a pipeline.
Try using your updateDoc inside a pipeline, like this:

var filter = { requestid: data.requestid };
var updateDoc = { $set: { requeststatus: { $concat: ["$requeststatus","-",`. ${data.status}`] } } };
var options = { multi: true };
var jobDetails = { filter, [updateDoc], options };

See how it works on the playground example