The answer
It runs JavaScript in the database for each document. Slow and a security risk. There is almost always a better way to write the query with other operators. Avoid $where.
MongoDB Query Operators for Node.js Developers
Why avoid $where in MongoDB?
More FAQs in MongoDB Query Operators for Node.js Developers
Comparison ($eq, $ne, $gt, $gte, $lt, $lte, $in, $nin, $exists), logical ($and, $or, $nor, $not), element ($exists, $type), array ($all, $elemMatch, $size), and evaluation ($regex, $text, $where).
Use $gte and $lte with Date objects: User.find({ createdAt: { $gte: new Date('2025-01-01'), $lt: new Date('2025-02-01') } }). Always use Date objects, not strings.
User.find({ skills: 'node' }). For multiple values, use $all: User.find({ skills: { $all: ['node', 'react'] } }). For at least one of multiple, use $in: User.find({ skills: { $in: ['node', 'react'] } }).
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