Database Manual / Reference / Query Language / Expressions

$isArray (expression operator)

Definition

$isArray

Determines if the operand is an array. Returns a boolean.

$isArray has the following syntax:

{ $isArray: [ <expression> ] }

Behavior

The <expression> can be any valid expression. For more information on expressions, see Expressions.

ExampleResultsNotes

{ $isArray: "hello" }

false

"hello" is a string, passed as a string.

{ $isArray: [ "hello" ] }

false

"hello" is a string, passed as part of an argument array.

{ $isArray: [ [ "hello" ] ] }

true

[ "hello" ] is an array, passed as part of an argument array.

Note

Aggregation expressions accept a variable number of arguments. These arguments are normally passed as an array. However, when the argument is a single value, you can simplify your code by passing the argument directly without wrapping it in an array.

Example

Create a collection named warehouses with the following documents:

db.warehouses.insertMany( [
   { _id : 1, instock: [ "chocolate" ], ordered: [ "butter", "apples" ] },
   { _id : 2, instock: [ "apples", "pudding", "pie" ] },
   { _id : 3, instock: [ "pears", "pecans" ], ordered: [ "cherries" ] },
   { _id : 4, instock: [ "ice cream" ], ordered: [ ] }
] )

Check if the instock and the ordered fields are arrays. If both fields are arrays, concatenate them:

db.warehouses.aggregate( [
   { $project:
      { items:
          { $cond:
            {
              if: { $and: [ { $isArray: "$instock" },
                            { $isArray: "$ordered" }
                          ] },
              then: { $concatArrays: [ "$instock", "$ordered" ] },
              else: "One or more fields is not an array."
            }
          }
      }
   }
] )
{ _id : 1, items : [ "chocolate", "butter", "apples" ] }
{ _id : 2, items : "One or more fields is not an array." }
{ _id : 3, items : [ "pears", "pecans", "cherries" ] }
{ _id : 4, items : [ "ice cream" ] }

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