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

1-js/02-first-steps/14-switch/article.md

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+# The "switch" statement
+
+A `switch` statement can replace multiple `if` checks.
+
+It gives a more descriptive way to compare a value with multiple variants.
+
+## The syntax
+
+The `switch` has one or more `case` blocks and an optional default.
+
+It looks like this:
+
+```js no-beautify
+switch(x) {
+  case 'value1':  // if (x === 'value1')
+    ...
+    [break]
+
+  case 'value2':  // if (x === 'value2')
+    ...
+    [break]
+
+  default:
+    ...
+    [break]
+}
+```
+
+- The value of `x` is checked for a strict equality to the value from the first `case` (that is, `value1`) then to the second (`value2`) and so on.
+- If the equality is found, `switch` starts to execute the code starting from the corresponding `case`, until the nearest `break` (or until the end of `switch`).
+- If no case is matched then the `default` code is executed (if it exists).
+
+## An example
+
+An example of `switch` (the executed code is highlighted):
+
+```js run
+let a = 2 + 2;
+
+switch (a) {
+  case 3:
+    alert( 'Too small' );
+    break;
+*!*
+  case 4:
+    alert( 'Exactly!' );
+    break;
+*/!*
+  case 5:
+    alert( 'Too large' );
+    break;
+  default:
+    alert( "I don't know such values" );
+}
+```
+
+Here the `switch` starts to compare `a` from the first `case` variant that is `3`. The match fails.
+
+Then `4`. That's a match, so the execution starts from `case 4` until the nearest `break`.
+
+**If there is no `break` then the execution continues with the next `case` without any checks.**
+
+An example without `break`:
+
+```js run
+let a = 2 + 2;
+
+switch (a) {
+  case 3:
+    alert( 'Too small' );
+*!*
+  case 4:
+    alert( 'Exactly!' );
+  case 5:
+    alert( 'Too big' );
+  default:
+    alert( "I don't know such values" );
+*/!*
+}
+```
+
+In the example above we'll see sequential execution of three `alert`s:
+
+```js
+alert( 'Exactly!' );
+alert( 'Too big' );
+alert( "I don't know such values" );
+```
+
+````smart header="Any expression can be a `switch/case` argument"
+Both `switch` and `case` allow arbitrary expressions.
+
+For example:
+
+```js run
+let a = "1";
+let b = 0;
+
+switch (+a) {
+*!*
+  case b + 1:
+    alert("this runs, because +a is 1, exactly equals b+1");
+    break;
+*/!*
+
+  default:
+    alert("this doesn't run");
+}
+```
+Here `+a` gives `1`, that's compared with `b + 1` in `case`, and the corresponding code is executed.
+````
+
+## Grouping of "case"
+
+Several variants of `case` which share the same code can be grouped.
+
+For example, if we want the same code to run for `case 3` and `case 5`:
+
+```js run no-beautify
+let a = 3;
+
+switch (a) {
+  case 4:
+    alert('Right!');
+    break;
+
+*!*
+  case 3: // (*) grouped two cases
+  case 5:
+    alert('Wrong!');
+    alert("Why don't you take a math class?");
+    break;
+*/!*
+
+  default:
+    alert('The result is strange. Really.');
+}
+```
+
+Now both `3` and `5` show the same message.
+
+The ability to "group" cases is a side-effect of how `switch/case` works without `break`. Here the execution of `case 3` starts from the line `(*)` and goes through `case 5`, because there's no `break`.
+
+## Type matters
+
+Let's emphasize that the equality check is always strict. The values must be of the same type to match.
+
+For example, let's consider the code:
+
+```js run
+let arg = prompt("Enter a value?");
+switch (arg) {
+  case '0':
+  case '1':
+    alert( 'One or zero' );
+    break;
+
+  case '2':
+    alert( 'Two' );
+    break;
+
+  case 3:
+    alert( 'Never executes!' );
+    break;
+  default:
+    alert( 'An unknown value' );
+}
+```
+
+1. For `0`, `1`, the first `alert` runs.
+2. For `2` the second `alert` runs.
+3. But for `3`, the result of the `prompt` is a string `"3"`, which is not strictly equal `===` to the number `3`. So we've got a dead code in `case 3`! The `default` variant will execute.