$last has the following syntax: { $last: }.

The Boolean expression evaluates all other values as true, including non-zero numeric values and arrays.

However, since $ is equivalent to $$CURRENT., rebinding CURRENT changes the meaning of $ accesses.

$let defines variables for use within the scope of a subexpression and returns the result of the subexpression.

$group consumes all input documents and outputs one document per each distinct group.

If low is not defined outside this $let expression block, the expression is invalid.

The mongo shell iterates the returned cursor automatically to print the results.

Other pipeline operations provide tools for grouping and sorting documents by specific field.

Finalize fucntion is optional for mapreduce operation.

$let defines variables for use within the scope of a subexpression and returns the result of the subexpression.