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 solution
-```js run demo
 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() {
@@ -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() {
 

1-js/05-data-types/03-string/3-truncate/solution.md

@@ -2,10 +2,9 @@ The maximal length must be `maxlength`, so we need to cut it a little shorter, t
 
 Note that there is actually a single unicode character for an ellipsis. That's not three dots.
 
-```js run
+```js run demo
 function truncate(str, maxlength) {
   return (str.length > maxlength) ?
     str.slice(0, maxlength - 1) + '…' : str;
 }
 ```
-

1-js/05-data-types/03-string/4-extract-currency/solution.md

@@ -1,5 +0,0 @@
-```js run
-    function extractCurrencyValue(str) {
-      return +str.slice(1);
-    }
-```

1-js/05-data-types/04-array/10-maximal-subarray/solution.md

@@ -1,4 +1,4 @@
-# The slow solution
+# Slow solution
 
 We can calculate all possible subsums.
 
@@ -67,7 +67,7 @@ Let's walk the array and keep the current partial sum of elements in the variabl
 
 If the description is too vague, please see the code, it's short enough:
 
-```js run
+```js run demo
 function getMaxSubSum(arr) {
   let maxSum = 0;
   let partialSum = 0;

1-js/05-data-types/05-array-methods/1-camelcase/_js.view/solution.js

@@ -1,8 +1,10 @@
 function camelize(str) {
   return str
-    .split('-') // my-long-word -> ['my', 'long', 'word']
+    .split('-') // splits 'my-long-word' into array ['my', 'long', 'word']
     .map(
+      // capitalizes first letters of all array items except the first one
+      // converts ['my', 'long', 'word'] into ['my', 'Long', 'Word']
       (word, index) => index == 0 ? word : word[0].toUpperCase() + word.slice(1)
-    ) // ['my', 'long', 'word'] -> ['my', 'Long', 'Word']
+    )
-    .join(''); // ['my', 'Long', 'Word'] -> myLongWord
+    .join(''); // joins ['my', 'Long', 'Word'] into 'myLongWord'
 }

1-js/05-data-types/05-array-methods/11-array-unique/solution.md

@@ -2,7 +2,7 @@ Let's walk the array items:
 - For each item we'll check if the resulting array already has that item.
 - If it is so, then ignore, otherwise add to results.
 
-```js run
+```js run demo
 function unique(arr) {
   let result = [];
 

1-js/05-data-types/05-array-methods/2-filter-range/solution.md

@@ -1,4 +1,4 @@
-``` js run
+```js run demo
 function filterRange(arr, a, b) {
   // added brackets around the expression for better readability
   return arr.filter(item => (a <= item && item <= b));

1-js/05-data-types/05-array-methods/3-filter-range-in-place/_js.view/solution.js

@@ -1,5 +1,4 @@
 
-
 function filterRangeInPlace(arr, a, b) {
 
   for (let i = 0; i < arr.length; i++) {

1-js/05-data-types/05-array-methods/3-filter-range-in-place/solution.md

@@ -1,4 +1,4 @@
-``` js run
+```js run demo
 function filterRangeInPlace(arr, a, b) {
 
   for (let i = 0; i < arr.length; i++) {
@@ -18,5 +18,4 @@ let arr = [5, 3, 8, 1];
 filterRangeInPlace(arr, 1, 4); // removed the numbers except from 1 to 4
 
 alert( arr ); // [3, 1]
-
 ```

1-js/05-data-types/07-map-set-weakmap-weakset/02-filter-anagrams/solution.md

@@ -59,7 +59,7 @@ Here we could also use a plain object instead of the `Map`, because keys are str
 
 That's how the solution can look:
 
-```js run
+```js run demo
 function aclean(arr) {
   let obj = {};
 

1-js/05-data-types/08-keys-values-entries/01-sum-salaries/solution.md

@@ -1,4 +1,4 @@
-```js run
+```js run demo
 function sumSalaries(salaries) {
 
   let sum = 0;
@@ -8,7 +8,22 @@ function sumSalaries(salaries) {
 
   return sum; // 650
 }
+
+let salaries = {
+  "John": 100,
+  "Pete": 300,
+  "Mary": 250
+};
+
+alert( sumSalaries(salaries) ); // 650
 ```
 Or, optionally, we could also get the sum using `Object.values` and `reduce`:
 
-`Object.values(salaries).reduce((a, b) => a + b) // 650`
+```js
+// reduce loops over array of salaries,
+// adding them up
+// and returns the result
+function sumSalaries(salaries) {
+  return Object.values(salaries).reduce((a, b) => a + b, 0) // 650
+}
+```

1-js/05-data-types/10-date/2-get-week-day/solution.md

@@ -2,7 +2,7 @@ The method `date.getDay()` returns the number of the weekday, starting from sund
 
 Let's make an array of weekdays, so that we can get the proper day name by its number:
 
-```js run
+```js run demo
 function getWeekDay(date) {
   let days = ['SU', 'MO', 'TU', 'WE', 'TH', 'FR', 'SA'];
 

1-js/05-data-types/10-date/3-weekday/solution.md

@@ -1,14 +0,0 @@
-```js run
-function getLocalDay(date) {
-
-  let day = date.getDay();
-
-  if (day == 0) { // 0 becomes 7
-    day = 7;
-  }
-
-  return day;
-}
-
-alert( getLocalDay(new Date(2012, 0, 3)) ); // 2
-```

1-js/05-data-types/10-date/4-get-date-ago/solution.md

@@ -11,7 +11,7 @@ function getDateAgo(date, days) {
 
 To implement it let's clone the date, like this:
 
-```js run
+```js run demo
 function getDateAgo(date, days) {
   let dateCopy = new Date(date);
 

1-js/05-data-types/10-date/5-last-day-of-month/solution.md

@@ -1,5 +1,5 @@
 Let's create a date using the next month, but pass zero as the day:
-```js run
+```js run demo
 function getLastDayOfMonth(year, month) {
   let date = new Date(year, month + 1, 0);
   return date.getDate();

1-js/05-data-types/10-date/8-format-date-relative/solution.md

@@ -1,6 +1,6 @@
 To get the time from `date` till now -- let's substract the dates.
 
-```js run
+```js run demo
 function formatDate(date) {
   let diff = new Date() - date; // the difference in milliseconds
 

1-js/06-advanced-functions/03-closure/6-filter-through-function/solution.md

@@ -14,7 +14,7 @@ alert( arr.filter(inBetween(3, 6)) ); // 3,4,5,6
 
 # Filter inArray
 
-```js run
+```js run demo
 function inArray(arr) {
   return function(x) {
     return arr.includes(x);

1-js/06-advanced-functions/03-closure/8-make-army/solution.md

@@ -55,9 +55,9 @@ function makeArmy() {
 
 As a result, all `shooter` functions get from the outer lexical envrironment the same, last value `i=10`.
 
-The fix can be very simple:
+We can fix it by moving the variable definition into the loop:
 
-```js run
+```js run demo
 function makeArmy() {
 
   let shooters = [];
@@ -80,9 +80,9 @@ army[0](); // 0
 army[5](); // 5
 ```
 
-Now it works correctly, because every time the code block in `for (..) {...}` is executed, a new Lexical Environment is created for it, with the corresponding value of `i`.
+Now it works correctly, because every time the code block in `for (let i=0...) {...}` is executed, a new Lexical Environment is created for it, with the corresponding variable `i`.
 
-So, the value of `i` now lives a little bit closer. Not in `makeArmy()` Lexical Environment, but in the Lexical Environment that corresponds the current loop iteration. A `shooter` gets the value exactly from the one where it was created.
+So, the value of `i` now lives a little bit closer. Not in `makeArmy()` Lexical Environment, but in the Lexical Environment that corresponds the current loop iteration. That's why now it works.
 
 ![](lexenv-makearmy.png)
 
@@ -90,7 +90,6 @@ Here we rewrote `while` into `for`.
 
 Another trick could be possible, let's see it for better understanding of the subject:
 
-
 ```js run
 function makeArmy() {
   let shooters = [];

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 demo
-
-```js run no-beautify
 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,