Add the attribute 'key' to div

Top level elements inside arrays should have the 'key' attribute set to allow React to efficiently rerender the elements of the array.

14_Day_Component_Life_Cycles/14_component_life_cycles_boilerplate/src/index.js

@@ -29,9 +29,9 @@ class App extends Component {
       })
   }
   renderCountries = () => {
-    return this.state.data.map((country) => {
+    return this.state.data.map((country, i) => {
       return (
-        <div>
+        <div key={i}>
           <div>
             {' '}
             <img src={country.flag} alt={country.name} />{' '}