# Variables

{% hint style="info" %}
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 ($$); i.e. "$$".
{% endhint %}

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                                                                             |