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Aggregation Operators

$type (aggregation)

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  • Definition
  • Behavior
  • Example

Definition

$type

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

$type has the following operator expression syntax:

{ $type: <expression> }

The argument can be any valid expression.

Tip

See also:

  • $isNumber - checks if the argument is a number.

  • $type (Query) - filters fields based on BSON type.

Behavior

$type

Unlike the $type query operator, which matches array elements based on their BSON type, the $type aggregation operator does not examine array elements. Instead, when passed an array as its argument, the $type aggregation operator 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 several common types of expressions:

Example
Results
{ $type: "a" }
"string"
{ $type: /a/ }
"regex"
{ $type: 1 }
"double"
{ $type: NumberLong(627) }
"long"
{ $type: { x: 1 } }
"object"
{ $type: [ [ 1, 2, 3 ] ] }
"array"

Note

In the case of a literal array such as [ 1, 2, 3 ], enclose the expression in an outer set of array brackets to prevent MongoDB from parsing [ 1, 2, 3 ] as an argument list with three arguments (1, 2, 3). Wrapping the array [ 1, 2, 3 ] in a $literal expression achieves the same result.

See operator expression syntax forms for more information.

Available Types

Type
Number
Alias
Notes
Double
1
"double"
String
2
"string"
Object
3
"object"
Array
4
"array"
Binary data
5
"binData"
Undefined
6
"undefined"
Deprecated.
ObjectId
7
"objectId"
Boolean
8
"bool"
Date
9
"date"
Null
10
"null"
Regular Expression
11
"regex"
DBPointer
12
"dbPointer"
Deprecated.
JavaScript
13
"javascript"
Symbol
14
"symbol"
Deprecated.
32-bit integer
16
"int"
Timestamp
17
"timestamp"
64-bit integer
18
"long"
Decimal128
19
"decimal"
Min key
-1
"minKey"
Max key
127
"maxKey"

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 with the 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 $type operator to display the type of field a for all documents as 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" }

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