make sure a solution always shows up, use "demo" if exists

1-js/06-advanced-functions/09-call-apply-decorators/02-delay/solution.md

@@ -1,6 +1,6 @@
 The solution:
 
-```js
+```js run demo
 function delay(f, ms) {
 
   return function() {
@@ -8,20 +8,25 @@ function delay(f, ms) {
   };
 
 }
+
+let f1000 = delay(alert, 1000);
+
+f1000("test"); // shows "test" after 1000ms
 ```
 
 Please note how an arrow function is used here. As we know, arrow functions do not have own `this` and `arguments`, so `f.apply(this, arguments)` takes `this` and `arguments` from the wrapper.
 
-If we pass a regular function, `setTimeout` would call it without arguments and `this=window` (in-browser), so we'd need to write a bit more code to pass them from the wrapper:
+If we pass a regular function, `setTimeout` would call it without arguments and `this=window` (assuming we're in the browser).
+
+We still can pass the right `this` by using an intermediate variable, but that's a little bit more cumbersome:
 
 ```js
 function delay(f, ms) {
 
-  // added variables to pass this and arguments from the wrapper inside setTimeout
   return function(...args) {
-    let savedThis = this;
+    let savedThis = this; // store this into an intermediate variable
     setTimeout(function() {
-      f.apply(savedThis, args);
+      f.apply(savedThis, args); // use it here
     }, ms);
   };
 

1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/solution.md

@@ -1,6 +1,4 @@
-
-
-```js run no-beautify
+```js demo
 function debounce(f, ms) {
 
   let isCooldown = false;
@@ -18,7 +16,7 @@ function debounce(f, ms) {
 }
 ```
 
-The call to `debounce` returns a wrapper. There may be two states:
+A call to `debounce` returns a wrapper. There may be two states:
 
 - `isCooldown = false` -- ready to run.
 - `isCooldown = true` -- waiting for the timeout.

1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/solution.md

@@ -1,4 +1,4 @@
-```js
+```js demo
 function throttle(func, ms) {
 
   let isThrottled = false,