minor fixes

1-js/05-data-types/07-map-set/article.md

@@ -41,6 +41,12 @@ alert( map.size ); // 3
 
 As we can see, unlike objects, keys are not converted to strings. Any type of key is possible.
 
+```smart header="`map[key]` isn't the right way to use a `Map`"
+Although `map[key]` also works, e.g. we can set `map[key] = 2`, this is treating `map` as a plain JavaScript object, so it implies all corresponding limitations (no object keys and so on).
+
+So we should use `map` methods: `set`, `get` and so on.
+```
+
 **Map can also use objects as keys.**
 
 For instance: