MongoError unknown group operator
<p>I am using MongoDb Driver for NodeJS.</p>

<p>I am facing issues in using aggregation. 
The error is</p>

<blockquote>
  <p>{"name":"MongoError","message":"unknown group operator
  '_id'","ok":0,"errmsg":"unknown group operator '_id'","code":15952}</p>
</blockquote>

<p>for the below script : </p>

<pre><code>MongoClient.connect(url, function (err, db) {
      if (err) 
      {
        console.log('Unable to connect to the mongoDB server. Error:', err);
        return;
      }
    var collName = "order";
    var whereParas = {};
    var groupParas = {"_id":null,total:{$sum:"$Value"}};
    var havingParas = {};

            db.collection(collName).aggregate(
            [
            { $match: whereParas },
            {
              $group: { groupParas}
            },
            { $match: havingParas }
        ]).toArray(function (err,result) {
            console.log("err");
            console.log(err);
            console.log("result");
            console.log(result);

            });
});
</code></pre>

<p>The data used is <a href="https://i.stack.imgur.com/aKqvb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aKqvb.png" alt="enter image description here"></a></p>

<p>Desired Output is Sum of Values.</p>

<p>In sql, I would have written :</p>

<pre><code>Select Sum(Value) From order
</code></pre>