$type (aggregation)

Definition

• $type

New in version 3.4.

Returns a string that specifies the BSON type of the argument.

$type has the following operator expressionsyntax:

{ $type: <expression> }

The argument can be any valid expression.

See also

If you wish to filter documents by their BSON type rather than returning the type of an expression,use the $type query operator.

Behavior

$type

Unlike the $type query operator, which matches arrayelements based on their BSON type, the $typeaggregation operator does not examine array elements. Instead,when passed an array as its argument, the $type aggregationoperator returns the type of the argument, i.e. "array".

If the argument is a field that is missing in the input document,$type returns the string "missing".

The following table shows the $type output for severalcommon types of expressions:

Note

In the case of a literal array such as [ 1, 2, 3 ],enclose the expression in an outer setof array brackets to prevent MongoDB from parsing[ 1, 2, 3 ] as anargument listwith three arguments (1, 2, 3). Wrapping the array[ 1, 2, 3 ] in a $literal expressionachieves the same result.

See operator expression syntax forms for more information.

Available Types

If the argument is a field that is missing in the input document,$type returns the string "missing".

Example

This example uses a collection named coll withthe following documents:

{ _id: 0, a : 8 }
{ _id: 1, a : [ 41.63, 88.19 ] }
{ _id: 2, a : { a : "apple", b : "banana", c: "carrot" } }
{ _id: 3, a :  "caribou" }
{ _id: 4, a : NumberLong(71) }
{ _id: 5 }

The following aggregation operation uses the $typeoperator to display the type of field a for all documentsas part of the $project stage.

db.coll.aggregate([{
    $project: {
       a : { $type: "$a" }
    }
}])

The operation returns the following:

{ _id: 0, "a" : "double" }
{ _id: 1, "a" : "array" }
{ _id: 2, "a" : "object" }
{ _id: 3, "a" : "string" }
{ _id: 4, "a" : "long" }
{ _id: 5, "a" : "missing" }