In MongoDB, the $radiansToDegrees aggregation pipeline operator converts an input value measured in radians to degrees.

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

The $radiansToDegrees operator was introduced in MongoDB 4.2.

Example

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

{ "_id" : 1, "data" : 0.5 }

And let’s assume that the value of the data field is in radians (i.e. 0.5 radians).

We can use the $radiansToDegrees operator to convert the data field to degrees:

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

Result:

{ "radians" : 0.5, "degrees" : 28.64788975654116 }

By default, the $radiansToDegrees 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.

128-Bit Decimal Values

If the expression provided to $radiansToDegrees is 128-bit decimal, then the result is returned in 128-bit decimal.

Suppose we add the following document to our collection:

{ "_id" : 2, "data" : NumberDecimal("0.1301023541559787031443874490659") }

Let’s run the the $radiansToDegrees operator against that document:

db.test.aggregate(
  [
    { $match: { _id: 2 } },
    { $project: { 
        _id: 0,
        radians: "$data",
        degrees: { $radiansToDegrees: "$data" }
      }
    }
  ]
).pretty()

Result:

{
	"radians" : NumberDecimal("0.1301023541559787031443874490659"),
	"degrees" : NumberDecimal("7.454315797853905125952127312900524")
}

The output is 128-bit decimal.

Null Values

Null values return null when using the $radiansToDegrees operator.

Suppose we add the following document to our collection:

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

Let’s run the the $radiansToDegrees operator against that document:

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

Result:

{ "degrees" : null }

We can see that the result is null.

NaN Values

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

Example:

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

Result:

{ "degrees" : NaN }

Infinity

If the argument resolves to Infinity or -Infinity, the $radiansToDegrees operator returns Infinity.

Suppose we add the following document to our collection:

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

Let’s run $radiansToDegrees against the data field:

db.test.aggregate(
  [
    { $match: { _id: 4 } },
    { $project: { 
        _id: 0,
        degrees: { $radiansToDegrees: "$data" }
      }
    }
  ]
)

Result:

{ "degrees" : Infinity }

Non-Existent Fields

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

Example:

db.test.aggregate(
  [
    { $match: { _id: 4 } },
    { $project: { 
        _id: 0,
        degrees: { $radiansToDegrees: "$name" }
      }
    }
  ]
)

Result:

{ "degrees" : null }

Combined with Other Operators

Some aggregation pipeline operators return their result in radians. You can combine $radiansToDegrees with such operators to return the result in degrees.

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

{ "_id" : 1, "data" : 0.5 }

And suppose we want to use the $asin operator to return the arcsine of the data field. The $asin operator returns its result in radians, but suppose we want the result in degrees instead.

In this case, we can do the following:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        radians: { $asin: "$data" },
        degrees: { $radiansToDegrees: { $asin: "$data" } }
      }
    }
  ]
)

Result:

{ "radians" : 0.5235987755982988, "degrees" : 29.999999999999996 }

In this example, the first field presents the result in radians, and the second field presents it in degrees.