Variables

Aggregation expressions can use both user-defined and system variables.

Variables can hold any BSON type data. To access the value of the variable, prefix the variable name with double dollar signs ($); i.e. "$".

For example:

Variable	Access via $	Brief Description
close	$$CLOSE	returns the absolute value of a number
context	$$CONTEXT	returns a numeric value between 0 and π radians for x between -1 and 1
cursor	$$CURSOR	returns the inverse hyperbolic cosine of a number
date	$$DATE	N/A
env	$$ENV	N/A
now	$$NOW	A variable that returns the current datetime value
random	$$RANDOM	returns a floating-point, pseudo-random number in the range 0 to less than 1
remove	$$REMOVE	A variable which evaluates to the missing value
request	$$REQUEST	N/A
root	$$ROOT	References the root currently being processed in the aggregation pipeline stage
script	$$SCRIPT	N/A
var	$$VAR	N/A