updateState - a utility for representing/updating program state as an object.

example.js

var STATE_FILTERS = [
  function clampIndex(state, old, update) {
    return extend(state, {
      // Validate and sanitize properties.
      index: clamp(state.index, 0, state.units - 1)
    });
  },
  function calculateOffset(state, old, update) {
    return extend(state, {
      // Create calculated properties.
      offset: state.index * state.unitWidth
    });
  }
];

// Set up state.
var STATE = updateState({}, {
  index: 0,
  units: 9,
  unitWidth: 320
}, STATE_FILTERS);

el.addEventListener('click', function (event) {
  if (event.target === nextEl) {
    // Any time a change is made to program state, use updateState.
    STATE = render(updateState(STATE, { index: STATE.index + 1 }, STATE_FILTERS));
    event.preventDefault();
  }
  else if (event.target === prevEl) {
    STATE = render(updateState(STATE, { index: STATE.index - 1 }, STATE_FILTERS));
    event.preventDefault();
  }
});

updatestate.js

function lambda(method) {
  // Convert a method func that uses a `this` context into a function that takes
  // the context object as the first parameter.
  return function (context) {
    var args = Array.prototype.slice.call(arguments, 1);
    return method.apply(context, args);
  };
}

// A lambda approach to `Array.prototype.slice`.
// Used in a lot of places for slicing the arguments object into a proper array.
var slice = Array.slice || lambda(Array.prototype.slice);
var reduce = Array.reduce || lambda(Array.prototype.reduce);

function extend(obj/* obj1, obj2, objN */) {
  // Copy all keys and values of obj1..objN to obj.
  // Mutates obj. Tip: use `Object.create` to copy values to a new object.
  return reduce(slice(arguments, 1), function (obj, objN) {
    return reduce(Object.keys(objN), function (obj, key) {
      obj[key] = objN[key];
      return obj;
    }, obj);
  }, obj);
}

function extendState(state, old, update) {
  // This filter is always run first by updateState.
  // Extends state with keys/values from update 
  return extend(state, update);
}

function updateState(old, update, filters) {
  // Create a new "state" object from `old` by running it through a series of
  // filter functions.
  // Start by extending a prototypal copy of `old` with `update`.
  return reduce([extendState].concat(filters), function (state, filter) {
    // Filter functions receive current state, old state and update object.
    return filter(state, old, update);
  }, Object.create(old));
}