how to set initial state in redux

It needs to be the second argument to createStore:

const rootReducer = combineReducers({
  todos: todos,
  visibilityFilter: visibilityFilter
});

const initialState = { 
  todos: [{id:123, text:'hello', completed: false}] 
};

const store = createStore(
  rootReducer, 
  initialState
);

You can set the initial state in the reducer(s).

const initialTodos = [{id:123, text:'hello', completed: false}]

// this is the ES2015 syntax for setting a default value for state in the function parameters
function todoReducer(state = initialTodos, action) {
  switch(action.type) {
    ... 
  }
  return state
}


const todoApp = combineReducers({
  // todos now defaults to the array of todos that you wanted and will be updated when you pass a new set of todos to the todoReducer
  todos: todoReducer,
  visibilityFilter
})