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$type (aggregation)

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Definition

$type

New in version 3.4.

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

$type has the following operator expression syntax:

copy 
{ $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 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.
JavaScript (with scope) 15 “javascriptWithScope”  
32-bit integer 16 “int”  
Timestamp 17 “timestamp”  
64-bit integer 18 “long”  
Decimal128 19 “decimal” New in version 3.4.
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:

copy 
{ _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.

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

The operation returns the following:

copy 
{ _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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