[mongodb] String field value length in mongoDB

The data type of the field is String. I would like to fetch the data where character length of field name is greater than 40.

I tried these queries but returning error. 1.

db.usercollection.find(
{$where: "(this.name.length > 40)"}
).limit(2);

output :error: {
    "$err" : "TypeError: Cannot read property 'length' of undefined near '40)' ",
    "code" : 16722
}

this is working in 2.4.9 But my version is 2.6.5

Queries with $where and $expr are slow if there are too many documents.

Using $regex is much faster than $where, $expr.

db.usercollection.find({ 
  "name": /^[\s\S]{40,}$/, // name.length >= 40
})

or 

db.usercollection.find({ 
  "name": { "$regex": "^[\s\S]{40,}$" }, // name.length >= 40
})

This query is the same meaning with

db.usercollection.find({ 
  "$where": "this.name && this.name.length >= 40",
})

or

db.usercollection.find({ 
    "name": { "$exists": true },
    "$expr": { "$gte": [ { "$strLenCP": "$name" }, 40 ] } 
})

I tested each queries for my collection.

# find
$where: 10529.359ms
$expr: 5305.801ms
$regex: 2516.124ms

# count
$where: 10872.006ms
$expr: 2630.155ms
$regex: 158.066ms

This query will give both field value and length:

db.usercollection.aggregate([
{
    $project: {
        "name": 1,
        "length": { $strLenCP: "$name" }
    }} ])

Here is one of the way in mongodb you can achieve this.

db.usercollection.find({ $where: 'this.name.length < 4' })

I had a similar kind of scenario, but in my case string is not a 1st level attribute. It is inside an object. In here I couldn't find a suitable answer for it. So I thought to share my solution with you all(Hope this will help anyone with a similar kind of problem).

Parent Collection 

{
"Child":
{
"name":"Random Name",
"Age:"09"
}
}

Ex: If we need to get only collections that having child's name's length is higher than 10 characters.

 db.getCollection('Parent').find({$where: function() { 
for (var field in this.Child.name) { 
    if (this.Child.name.length > 10) 
        return true;

}
}})