The todos variable is being re-declared within the same block. Variables declared with const and let are block-scoped, meaning they don’t exist outside of the block they were called in. Blocks are created with {} brackets around if/else statements, loops, and functions.

There are many solutions, including changing the variable name in the different cases or removing the variable altogether. We can also wrap each of our cases in a block to isolate the variables.

function todoReducer(state, action) {
  switch (action.type) {
    case 'ADD_TODO': {
      const todos = state.todos
      return {
        ...state, todos:
        todos.concat(action.payload)
      }
    }
    case 'REMOVE_TODO': {
      const todos = state.todos
      const newTodos = todos.filter(
        (todo) => todo.id !== action.id
      )
      return { ...state, todos: newTodos }
    }
    default:
      return state
  }
}