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

1-js/02-first-steps/13-while-for/2-which-value-while/solution.md

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+The task demonstrates how postfix/prefix forms can lead to different results when used in comparisons.
+
+1. **From 1 to 4**
+
+    ```js run
+    let i = 0;
+    while (++i < 5) alert( i );
+    ```
+
+    The first value is `i = 1`, because `++i` first increments `i` and then returns the new value. So the first comparison is `1 < 5` and the `alert` shows `1`.
+
+    Then follow `2, 3, 4…` -- the values show up one after another. The comparison always uses the incremented value, because `++` is before the variable.
+
+    Finally, `i = 4` is incremented to `5`, the comparison `while(5 < 5)` fails, and the loop stops. So `5` is not shown.
+2. **From 1 to 5**
+
+    ```js run
+    let i = 0;
+    while (i++ < 5) alert( i );
+    ```
+
+    The first value is again `i = 1`. The postfix form of `i++` increments `i` and then returns the *old* value, so the comparison `i++ < 5` will use `i = 0` (contrary to `++i < 5`).
+
+    But the `alert` call is separate. It's another statement which executes after the increment and the comparison. So it gets the current `i = 1`.
+
+    Then follow `2, 3, 4…`
+
+    Let's stop on `i = 4`. The prefix form `++i` would increment it and use `5` in the comparison. But here we have the postfix form `i++`. So it increments `i` to `5`, but returns the old value. Hence the comparison is actually `while(4 < 5)` -- true, and the control goes on to `alert`.
+
+    The value `i = 5` is the last one, because on the next step `while(5 < 5)` is false.