What do expressions produce?
An expression can be an operator (e.g., {$concat: ...}), a variable (e.g., "$$ROOT"), or a field path (e.g., "$address"). In all these cases, an expression is just something that dynamically populates and returns a new element, which can be one of the following types:
- Number (including integer, long, float, double, and decimal128)
- String (UTF-8)
- Boolean
- DateTime (UTC)
- Array
- Object
However, a specific expression can restrict you to returning just one or a few of these types. For example, the {$concat: ...} operator, which combines multiple strings, can only produce a string data type (or null). The "$$ROOT" variable can only return an object that refers to the root document currently being processed in the pipeline stage.
A field path (e.g., "$address") is different and can return an element of any data type, depending on what the field refers to in the current input document....
