reducer_primer.js

function addTodo(state, action){
  var new_state;
  if(action.type !== "ADD_TODO")
    return state;
  if(!state)
    new_state=[];
  else
    new_state=state.slice();
  new_state.push(action.item);
  return new_state;
}

var state0 = addTodo(null, {type:"ADD_TODO", item : "abc"});
console.log(state0); //["abc"]

var state1 = addTodo(state0, {type:"ADD_TODO", item : "xyz"});
console.log(state1); //["abc", "xyz"]