Add nullish coalescing to multiple articles, refactor operators, renumber the chapter

1-js/02-first-steps/15-function-basics/1-if-else-required/task.md

@@ -0,0 +1,38 @@
+importance: 4
+
+---
+
+# Is "else" required?
+
+The following function returns `true` if the parameter `age` is greater than `18`.
+
+Otherwise it asks for a confirmation and returns its result:
+
+```js
+function checkAge(age) {
+  if (age > 18) {
+    return true;
+*!*
+  } else {
+    // ...
+    return confirm('Did parents allow you?');
+  }
+*/!*
+}
+```
+
+Will the function work differently if `else` is removed?
+
+```js
+function checkAge(age) {
+  if (age > 18) {
+    return true;
+  }
+*!*
+  // ...
+  return confirm('Did parents allow you?');
+*/!*
+}
+```
+
+Is there any difference in the behavior of these two variants?