$boolean(arg)

Casts the argument to a Boolean using the following rules:

Argument type Result
Boolean unchanged
string: empty false
string: non-empty true
number: 0 false
number: non-zero true
null false
array: empty false
array: contains a member that casts to true true
array: all members cast to false false
object: empty false
object: non-empty true
function false

$not(arg)

Returns Boolean NOT on the argument. arg is first cast to a boolean

$exists(arg)

Returns Boolean true if the arg expression evaluates to a value, or false if the expression does not match anything (e.g. a path to a non-existent field reference).