---
title: Use the Spread Operator on Arrays
---
## Use the Spread Operator on Arrays
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## Solution

```javascript
const immutableReducer = (state = ['Do not mutate state!'], action) => {
  switch(action.type) {
    case 'ADD_TO_DO':
      // don't mutate state here or the tests will fail
      let arr = [...state, action.todo];
      return arr;
    default:
      return state;
  }
};

const addToDo = (todo) => {
  return {
    type: 'ADD_TO_DO',
    todo
  }
}

const store = Redux.createStore(immutableReducer);
```