$in

The $in operator in MongoDB is used to match any one of the values specified in an array. It can be used with a field that contains an array or with a field that holds a scalar value. This operator is handy when you want to filter documents based on multiple possible values for a specific field.

Syntax

Here's the general structure of a query using the $in operator:

{ field: { $in: [<value1>, <value2>, ...] } }

Example

Consider a collection articles with the following documents:

[
  { _id: 1, title: 'MongoDB', tags: ['database', 'NoSQL'] },
  { _id: 2, title: 'Node.js', tags: ['javascript', 'runtime'] },
  { _id: 3, title: 'React', tags: ['library', 'javascript'] },
];

Let's say you want to find all articles that have either the "NoSQL" or "javascript" tag. You can use the $in operator like so:

db.articles.find({ tags: { $in: ['NoSQL', 'javascript'] } });

This will return the following documents:

[
  { _id: 1, title: 'MongoDB', tags: ['database', 'NoSQL'] },
  { _id: 2, title: 'Node.js', tags: ['javascript', 'runtime'] },
  { _id: 3, title: 'React', tags: ['library', 'javascript'] },
];

In conclusion, the $in operator allows you to specify an array of values and filter documents based on whether their field value exists within that array.