In one of the initial posts of the series, OP mentions that functional programming doesn't have app state. Can anyone point out more details on this? I don't k

In one of the initial posts of the series, OP mentions that functional programming doesn't have app state. Can anyone point out more details on this? I don't know if I am understanding it correctly but one of the reasons I absolutely loved Backbone.js was that it kept state of our DOM. Is this relevant to above discussion? because state surely looks pretty beneficial.