One solution from ES6 to help enforce state immutability in Redux is the spread operator: <code>...</code>. The spread operator has a variety of applications, one of which is well-suited to the previous challenge of producing a new array from an existing array. This is relatively new, but commonly used syntax. For example, if you have an array <code>myArray</code> and write:
<code>let newArray = [...myArray];</code>
<code>newArray</code> is now a clone of <code>myArray</code>. Both arrays still exist separately in memory. If you perform a mutation like <code>newArray.push(5)</code>, <code>myArray</code> doesn't change. The <code>...</code> effectively <i>spreads</i> out the values in <code>myArray</code> into a new array. To clone an array but add additional values in the new array, you could write <code>[...myArray, 'new value']</code>. This would return a new array composed of the values in <code>myArray</code> and the string <code>'new value'</code> as the last value. The spread syntax can be used multiple times in array composition like this, but it's important to note that it only makes a shallow copy of the array. That is to say, it only provides immutable array operations for one-dimensional arrays.
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## Instructions
<sectionid='instructions'>
Use the spread operator to return a new copy of state when a to-do is added.
- text: The Redux store should exist and initialize with a state equal to <code>[Do not mutate state!]</code>.
testString: assert((function() { const initialState = store.getState(); return ( Array.isArray(initialState) === true && initialState[0] === 'Do not mutate state!'); })(), 'The Redux store should exist and initialize with a state equal to <code>[Do not mutate state!]</code>.');
testString: assert(typeof addToDo === 'function' && typeof immutableReducer === 'function', '<code>addToDo</code> and <code>immutableReducer</code> both should be functions.');
testString: assert((function() { const initialState = store.getState(); const isFrozen = DeepFreeze(initialState); store.dispatch(addToDo('__TEST__TO__DO__')); const finalState = store.getState(); const expectedState = [ 'Do not mutate state!', '__TEST__TO__DO__' ]; return( isFrozen && DeepEqual(finalState, expectedState)); })(), 'Dispatching an action of type <code>ADD_TO_DO</code> on the Redux store should add a <code>todo</code> item and should NOT mutate state.');