Functional, with imperative boundary in js.

immutable_copy.js

var state = {
  value: 0
};

var f = function(currentState) {
  var newState = copy(currentState); ///copy is magic deep-copy function
  newState.value += 1;
  return newState;
};

var g = function(f) {
  state = f(state);
};
//f is pure, g applies state changes; 

immutable_f_returns_state_with_namespace.js

var state = {
  nameSpacedState: {
    value: 0
  }
};

var partialState = function(namespace, state) {
  return {
    namespace: namespace,
    state: state
  };
};

var f = function(allTheState) {
  return partialState(namespace, {
    value: scopeState(namespace, allTheState).value + 1
  });
};

var g = function(f) {
  var retVal = f(state);
  state[retVal.namespace] = retVal.state;
};

immutable_recreate.js

var state = {
  value: 0
};

var f = function(currentState) {
  return {
    value: currentState.value + 1
  }
};

var g = function(f) {
  state = f(state);
};
//f is pure, g applies state changes; 

mutable_state.js

var state = {
  value: 0
};

var f = function(currentState) {
  currentState.value += 1;
};

f(state);
//f has side-effects