Powershell has the upper hand here!

Still, bash can try to keep up using json.bash. :)

    $ source json.bash
    $ declare -A greeting=([Hello]=World)
    $ json ...@greeting:{}
    {"Hello":"World"}

... is splatting the greeting associative array entries into the object created by the json call.

Without the ... the greeting would be a nested object. Probably more clear with multiple entries:

    $ declare -A greeting=([Hello]=World [How]="are you?")
    $ json @greeting:{}   
    {"greeting":{"Hello":"World","How":"are you?"}}

Vs:

    $ json ...@greeting:{}                                
    {"Hello":"World","How":"are you?"}