$indexOfArray (aggregation)

Definition

$indexOfArray

Searches an array for an occurrence of a specified value and returns the array index (zero-based) of the first occurrence. If the value is not found, returns -1.

$indexOfArray has the following operator expression syntax:

{ $indexOfArray: [ <array expression>, <search expression>, <start>, <end> ] }

Field	Type	Description
<array>	string	Can be any valid expression as long as it resolves to an array. For more information on expressions, see Expressions. If the array expression resolves to a value of null or refers to a field that is missing, $indexOfArray returns null. If the array expression does not resolve to an array or null nor refers to a missing field, $indexOfArray returns an error.
<search value>	string	Can be any valid expression. For more information on expressions, see Expressions.
<start>	integer	Optional. An integer, or a number that can be represented as integers (such as 2.0), that specifies the starting index position for the search. Can be any valid expression that resolves to a non-negative integral number. If unspecified, the starting index position for the search is the beginning of the string.
<end>	integer	Optional. An integer, or a number that can be represented as integers (such as 2.0), that specifies the ending index position for the search. Can be any valid expression that resolves to a non-negative integral number. If you specify a <end> index value, you should also specify a <start> index value; otherwise, $indexOfArray uses the <end> value as the <start> index value instead of the <end> value. If unspecified, the ending index position for the search is the end of the string.

Behavior

If the <search expression> is found multiple times within the <array expression>, then $indexOfArray returns the index of the first <search expression> from the starting index position.

$indexOfArray returns null:

• If <array expression> is null, or

• If <array expression> refers to a non-existing field in the input document.

$indexOfArray returns an error:

• If <array expression> is not an array and not null, or

• If <start> or <end> is a negative integer (or a value that can be represented as a negative integer, like -5.0).

$indexOfArray returns -1:

• If the <search expression> is not found in the array, or

• If <start> is a number greater than <end>, or

• If <start> is a number greater than the length of the array.

Examples

Consider an inventory collection with the following documents:

{ "_id" : 1, "items" : ["one", "two", "three"] }
{ "_id" : 2, "items" : [1, 2, 3] }
{ "_id" : 3, "items" : [null, null, 2] }
{ "_id" : 4, "items" : null }
{ "_id" : 5, "amount" : 3 }

The following operation uses the $indexOfArray operator to return the array index at which the string foo is located in each items array:

db.inventory.aggregate(
   [
     {
       $project:
          {
            index: { $indexOfArray: [ "$items", 2 ] },
          }
      }
   ]
)

The operation returns the following results:

{ "_id" : 1, "index" : "-1" }
{ "_id" : 2, "index" : "1" }
{ "_id" : 3, "index" : "2" }
{ "_id" : 4, "index" : null }
{ "_id" : 5, "index" : null }