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

1-js/02-first-steps/15-function-basics/4-pow/solution.md

@@ -0,0 +1,21 @@
+
+```js run demo
+function pow(x, n) {
+  let result = x;
+
+  for (let i = 1; i < n; i++) {
+    result *= x;
+  }
+
+  return result;
+}
+
+let x = prompt("x?", '');
+let n = prompt("n?", '');
+
+if (n < 1) {
+  alert(`Power ${n} is not supported, use a positive integer`);
+} else {
+  alert( pow(x, n) );
+}
+```