refactor classes, add private, minor fixes

1-js/11-async/03-promise-chaining/01-then-vs-catch/task.md

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+# Promise: then versus catch
+
+Are these code fragments equal? In other words, do they behave the same way in any circumstances, for any handler functions?
+
+```js
+promise.then(f1).catch(f2);
+```
+
+Versus:
+
+```js
+promise.then(f1, f2);
+```