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

1-js/04-object-basics/01-object/3-is-empty/solution.md

@@ -1,10 +1 @@
 Just loop over the object and `return false` immediately if there's at least one property.
-
-```js
-function isEmpty(obj) {
-  for (let key in obj) {
-    return false;
-  }
-  return true;
-}
-```

1-js/04-object-basics/01-object/8-multiply-numeric/solution.md

@@ -1,19 +0,0 @@
-``` js run
-
-// before the call
-let menu = {
-  width: 200,
-  height: 300,
-  title: "My menu"
-};
-
-function multiplyNumeric(obj) {
-  for (let key in obj) {
-    if (typeof obj[key] == 'number') {
-      obj[key] *= 2;
-    }
-  }
-}
-
-alert(menu); 
-```

1-js/04-object-basics/04-object-methods/7-calculator/solution.md

@@ -1,6 +1,5 @@
 
-
-```js run demo
+```js run demo solution
 let calculator = {
   sum() {
     return this.a + this.b;
@@ -20,4 +19,3 @@ calculator.read();
 alert( calculator.sum() );
 alert( calculator.mul() );
 ```
-

1-js/04-object-basics/04-object-methods/8-chain-calls/solution.md

@@ -1,6 +1,6 @@
 The solution is to return the object itself from every call.
 
-```js run
+```js run demo
 let ladder = {
   step: 0,
   up() {
@@ -28,7 +28,7 @@ ladder.up().up().down().up().down().showStep(); // 1
 
 We also can write a single call per line. For long chains it's more readable:
 
-```js 
+```js
 ladder
   .up()
   .up()
@@ -37,4 +37,3 @@ ladder
   .down()
   .showStep(); // 1
 ```
-

1-js/04-object-basics/06-constructor-new/2-calculator-constructor/solution.md

@@ -1,5 +1,3 @@
-
-
 ```js run demo
 function Calculator() {