$match in mongoose aggregate query

I have a Schema defined as

const mongoose = require('mongoose');

const urlSchema = new mongoose.Schema({
    longURL : {
        type : String,
        required : true,
    },
    shortUrl: {
        type : String
    },
    clicks : {
        type : Number,
        default : 0
    },
    date: {type: Date, default: new Date()},

    year : {
        type : Number
    }
    
})

module.exports = mongoose.model('Url',urlSchema)

When I give default value for year in $match for the below code, I'm getting desired output.

let result = await Url.aggregate(
            [
                 {$match:{"year":2020}}, 
                
                {
                        $project:
                        {
                            month: { $month: "$date" }
                        }
        
                },
                {
                    $group : {
                        _id : "$month",
                        count : {$sum : 1}
                    }
                }
            ]
        )

output : 

[
    {
        "_id": 1,   // 1 - represent Jan
        "count": 8
    },
    {
         "_id: : 7,  // 7 - represent Jul
         "count: : 5
    }
]

But if I pass dynamic values, I'm getting empty array as output

let year = 2020;
let result = await Url.aggregate(
            [
                 {$match:{"year":year}}, 
                
                {
                        $project:
                        {
                            month: { $month: "$date" }
                        }
        
                },
                {
                    $group : {
                        _id : "$month",
                        count : {$sum : 1}
                    }
                }
            ]
        )

output: 
[]

Just started on working with mongoose and I don't find any reference why I'm getting this kind of weird result? I need count values based on year and month. Can anyone help me out of this?

Thanks for the help in advance!