A valid expression that $reduce applies to each element in the input array in left-to-right order. Wrap the input value with $reverseArray to yield the equivalent of applying the combining expression from right-to-left.
During evaluation of the in expression, two variables will be available:
value is the variable that represents the cumulative value of the expression.this is the variable that refers to the element being processed.The initial cumulative value set before in is applied to the first element of the input array.
Can be any valid expression that resolves to an array. For more information on expressions, see Expressions.
If the argument resolves to a value of null or refers to a missing field, $reduce returns null.
If the argument does not resolve to an array or null nor refers to a missing field, $reduce returns an error.
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Applies an expression to each element in an array and combines them into a single value.