minor fixes

1-js/05-data-types/03-string/article.md

@@ -445,6 +445,8 @@ There are 3 methods in JavaScript to get a substring: `substring`, `substr` and
     alert( str.substr(-4, 2) ); // 'gi', from the 4th position get 2 characters
     ```
 
+    This method resides in the [Annex B](https://tc39.es/ecma262/#sec-string.prototype.substr) of the language specification. It means that only browser-hosted Javascript engines should support it, and it's not recommended to use it. In practice, it's supported everywhere.
+
 Let's recap these methods to avoid any confusion:
 
 | method | selects... | negatives |

7-animation/3-js-animation/text.view/index.html

@@ -29,7 +29,7 @@ And stood awhile in thought.
         timing: bounce,
         draw: function(progress) {
           let result = (to - from) * progress + from;
-          textArea.value = text.substr(0, Math.ceil(result))
+          textArea.value = text.slice(0, Math.ceil(result))
         }
       });
     }