In MongoDB, the $tanh aggregation pipeline operator returns the hyperbolic tangent of a value that is measured in radians.

$tanh accepts any valid expression that resolves to a number.

The $tanh operator was introduced in MongoDB 4.2.

Example

Suppose we have a collection called test with the following document:

{ "_id" : 1, "data" : 2 }

We can use the $tanh operator to return the hyperbolic tangent of the data field:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        hyperbolicTangent: { $tanh: "$data" }
      }
    }
  ]
)

Result:

{ "hyperbolicTangent" : 0.9640275800758169 }

Convert to Radians

As mentioned, $tanh returns the hyperbolic tangent of a value that is measured in radians. If the value is in degrees, you can use the $degreesToRadians operator to convert it to radians.

Example:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        hyperbolicTangent: { $degreesToRadians: { $tanh: "$data" } }
      }
    }
  ]
)

Result:

{ "hyperbolicTangent" : 0.016825455352356293 }

128-Bit Decimal Values

By default, the $tanh operator returns values as a double, but it can also return values as a 128-bit decimal as long as the expression resolves to a 128-bit decimal value.

Suppose we add the following document to our collection:

{ "_id" : 2, "data" : NumberDecimal("2.1301023541559787031443874490659") }

Let’s apply the the $tanh operator against the data field in that document:

db.test.aggregate(
  [
    { $match: { _id: 2 } },
    { $project: { 
        _id: 0,
        result: { $tanh: "$data" }
      }
    }
  ]
)

Result:

{ "result" : NumberDecimal("0.9721543408207801550541565157881927") }

The output is 128-bit decimal.

Null Values

Null values return null when using the $tanh operator.

Suppose we add the following document to our collection:

{ "_id" : 3, "data" : null }

Let’s apply the the $tanh operator against that document:

db.test.aggregate(
  [
    { $match: { _id: 3 } },
    { $project: { 
        _id: 0,
        result: { $tanh: "$data" }
      }
    }
  ]
)

Result:

{ "result" : null }

We can see that the result is null.

NaN Values

If the argument resolves to NaN, $tanh returns NaN.

Example:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        result: { $tanh: 1 * "$data" }
      }
    }
  ]
)

Result:

{ "result" : NaN }

Non-Existent Fields

If the $tanh operator is applied against a field that doesn’t exist, null is returned.

Example:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        result: { $tanh: "$name" }
      }
    }
  ]
)

Result:

{ "result" : null }

Infinity

Providing Infinity returns 1 and providing -Infinity returns -1.

Suppose we add the following documents to the collection:

{ "_id" : 4, "data" : Infinity }
{ "_id" : 5, "data" : -Infinity }

Let’s apply $tanh to these documents:

db.test.aggregate(
  [
    { $match: { _id: { $in: [ 4, 5 ] } } },
    { $project: { 
        hyperbolicTangent: { $tanh: "$data" }
      }
    }
  ]
)

Result:

{ "_id" : 4, "hyperbolicTangent" : 1 }
{ "_id" : 5, "hyperbolicTangent" : -1 }