You set the custom flow variables using context.setVariable , Retrieve the flow variables using context.getVariable.
context.setVariable(),context.getVariable() are javascript functions that takes flow variable name as as string input & returns the value of that flow variable.
As far as i know, There is nothing like flow.getVariable/setVariable functions in javascript. I am not sure where you found these functions.
Ideally, You should be doing,
var a=context.getVariable("response.content");
Please refer our docs for more details. Hope it helps.