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Use the $where operator to pass either a string containing a JavaScript expression or a full JavaScript function to the query system. The $where provides greater flexibility, but requires that the database processes the JavaScript expression or function for each document in the collection. Reference the document in the JavaScript expression or function using either this or obj .


  • Do not write to the database within the $where JavaScript function.
  • $where evaluates JavaScript and cannot take advantage of indexes. Therefore, query performance improves when you express your query using the standard MongoDB operators (e.g., $gt, $in).
  • In general, you should use $where only when you can’t express your query using another operator. If you must use $where, try to include at least one other standard query operator to filter the result set. Using $where alone requires a table scan.

Consider the following examples:

db.myCollection.find( { $where: "this.credits == this.debits" } );
db.myCollection.find( { $where: "obj.credits == obj.debits" } );

db.myCollection.find( { $where: function() { return (this.credits == this.debits) } } );
db.myCollection.find( { $where: function() { return obj.credits == obj.debits; } } );

Additionally, if the query consists only of the $where operator, you can pass in just the JavaScript expression or JavaScript functions, as in the following examples:

db.myCollection.find( "this.credits == this.debits || this.credits > this.debits" );

db.myCollection.find( function() { return (this.credits == this.debits || this.credits > this.debits ) } );

You can include both the standard MongoDB operators and the $where operator in your query, as in the following examples:

db.myCollection.find( { active: true, $where: "this.credits - this.debits < 0" } );
db.myCollection.find( { active: true, $where: function() { return obj.credits - obj.debits < 0; } } );

Using normal non-$where query statements provides the following performance advantages:

  • MongoDB will evaluate non-$where components of query before $where statements. If the non-$where statements match no documents, MongoDB will not perform any query evaluation using $where.
  • The non-$where query statements may use an index.
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