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

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

@@ -0,0 +1,19 @@
+importance: 4
+
+---
+
+# Function pow(x,n)
+
+Write a function `pow(x,n)` that returns `x` in power `n`. Or, in other words, multiplies `x` by itself `n` times and returns the result.
+
+```js
+pow(3, 2) = 3 * 3 = 9
+pow(3, 3) = 3 * 3 * 3 = 27
+pow(1, 100) = 1 * 1 * ...* 1 = 1
+```
+
+Create a web-page that prompts for `x` and `n`, and then shows the result of `pow(x,n)`.
+
+[demo]
+
+P.S. In this task the function should support only natural values of `n`: integers up from `1`.