$abs (expression operator)

Definition

$abs

Returns the absolute value of a number.

$abs has the following syntax:

{ $abs: <number> }

The <number> expression can be any valid expression as long as it resolves to a number. For more information on expressions, see Expressions.

Behavior

If the argument resolves to a value of null or refers to a field that is missing, $abs returns null. If the argument resolves to NaN, $abs returns NaN.

Example

A collection temperatureChange contains the following documents:

db.temperatureChange.insertMany( [
   { _id: 1, startTemp: 50, endTemp: 80 },
   { _id: 2, startTemp: 40, endTemp: 40 },
   { _id: 3, startTemp: 90, endTemp: 70 },
   { _id: 4, startTemp: 60, endTemp: 70 }
] )

The following example calculates the magnitude of difference between the startTemp and endTemp ratings:

db.temperatureChange.aggregate([
   {
      $project: { delta: { $abs: { $subtract: [ "$startTemp", "$endTemp" ] } } }
   }
])

The operation returns the following results:

{ "_id" : 1, "delta" : 30 }
{ "_id" : 2, "delta" : 0 }
{ "_id" : 3, "delta" : 20 }
{ "_id" : 4, "delta" : 10 }