JavaScript #echo and using JS variables?

[super_iunctum]

In JS if I want to get a server side variable I can use the following code :

variable = '<!--#echo var="my_server_variable"-->';

However, I need to be able to dynamically request the system variables and have no idea what the correct syntax is, for example :

check_this = "my_server_variable";
variable = '<!--#echo var="'+check_this+'"-->';

That doesn't work, does anyone know the correct syntax to use if I want to get a server side variable using #echo but instead of hard coding the variable name I can use a javascript variable ?