$abs (expression operator)（表达式运算符）

Definition定义

$abs

Returns the absolute value of a number.返回一个数字的绝对值。

$abs has the following syntax:具有以下语法：

{ $abs: <number> }

The <number> expression can be any valid expression as long as it resolves to a number. <number>表达式可以是任何有效的表达式，只要它解析为数字。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

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, $abs returns NaN.如果参数解析为null值或引用缺少的字段，则$abs将返回null。如果参数解析为NaN，$abs将返回NaN。

Example示例

A collection temperatureChange contains the following documents:集合temperatureChange（气温变化）包含以下文档：

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:以下示例计算startTemp和endTemp额定值之间的差异大小：

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

The operation returns the following results:该操作返回以下结果：

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