We are trying to build a large file (size>30MB) upload from Android and iOS phones, for that created an Express.js API for file upload.

Endpoint: user/upload

uploadFile:function(req,res){

    //upload files to AWS s3 asychrnous
    s3.upload(params,options,function(err, data) {
        //when upload complete
        //need a help on this part
        if(data){
          //Send a notification to client
        }
    });

    res.json({
        status:true,
        'message':'Upload ongoing'
    });
}

When user requests the API (user/upload) it immediately give this response:

{
  status:true,
  'message':'Upload ongoing'
}

Because the upload is asynchronous. So how can I notify the client when upload completes?