add solution to Redux never mutate state (#35590)

guide/english/certifications/front-end-libraries/redux/never-mutate-state/index.md

@@ -3,8 +3,56 @@ title: Never Mutate State
 ---
 ## Never Mutate State
 
-This is a stub. <a href='https://github.com/freecodecamp/guides/tree/master/src/pages/certifications/front-end-libraries/redux/never-mutate-state/index.md' target='_blank' rel='nofollow'>Help our community expand it</a>.
+The goal of this challenge is to return a new copy of state in reducer function because of state immutability in Redux.
 
-<a href='https://github.com/freecodecamp/guides/blob/master/README.md' target='_blank' rel='nofollow'>This quick style guide will help ensure your pull request gets accepted</a>.
+### Hint 1
+const means: it cannot change through re-assignment, and it cannot be re-declared.
+Since objects and arrays are mutable, you can add to it by index (array[3] = 3), by property (object.name="sam"), by extending (with various array methods)
 
-<!-- The article goes here, in GitHub-flavored Markdown. Feel free to add YouTube videos, images, and CodePen/JSBin embeds  -->
+### Hint 2
+.push() and .splice() directly modify the array
+
+### Hint 3
+.concat() doesn't modify array but just returns a new array
+
+### Hint 4
+.slice() doesn't modify array but just returns a new array
+
+### Hint 5
+spread operator [...array] doesn't modify array but just returns a new array
+
+### Solution
+```javascript
+const ADD_TO_DO = 'ADD_TO_DO';
+
+// A list of strings representing tasks to do:
+const todos = [
+  'Go to the store',
+  'Clean the house',
+  'Cook dinner',
+  'Learn to code',
+];
+
+const immutableReducer = (state = todos, action) => {
+  switch(action.type) {
+    case ADD_TO_DO:
+      // don't mutate state here or the tests will fail
+      
+      return todos.concat(action.todo)
+      // or return [...todos, action.todo]
+      
+    default:
+      return state;
+  }
+};
+
+// an example todo argument would be 'Learn React',
+const addToDo = (todo) => {
+  return {
+    type: ADD_TO_DO,
+    todo
+  }
+}
+
+const store = Redux.createStore(immutableReducer);
+```