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src/App.js

@@ -24,7 +24,8 @@ class BooksApp extends React.Component {
   onSelectChange = (e) => {
     const id = e.target.name
     const newShelf = e.target.value
-    const books = this.state.books.slice(); // Create local copy to change.
+
+    const books = this.state.books.slice();
     books.forEach((book) => {
       if (book.id === id) {
         book.shelf = newShelf;