Basic JS: Iterating through arrays with for. Replace stub with guide (#18527)

Guide for [Basic JavaScript: Iterate Through an Array with a For Loop](https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/basic-javascript/iterate-through-an-array-with-a-for-loop)

client/src/pages/guide/english/certifications/javascript-algorithms-and-data-structures/basic-javascript/iterate-through-an-array-with-a-for-loop/index.md

@@ -2,9 +2,73 @@
 title: Iterate Through an Array with a For Loop
 ---
 ## Iterate Through an Array with a For Loop
+### Problem explanation:
+_Declare and initialize a variable `total` to `0`. Use a `for` loop to add the value of each element of the `myArr` array to `total`._
 
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+#### Hint 1
+Remember the structure of a `for` loop:
+`for ([initialization]; [condition]; [final-expression])
+   statement`
    
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+- The `[initialization]` part is executed only once (the first time).
+- The `[condition]` is checked on every iteration.
+- The `[final-expression]` is executed along the `statement` if `[condition]` resolves to `true`.
+> _try to solve the problem now_
+
+
+## Spoiler alert!
+
+**Solution ahead!**
+
+## Code solution:
+
+```javascript
+for (var i = 0; i < myArr.length; i++) {
+  total += myArr[i];
+}
+```
+·  Run code at [repl.it](https://repl.it/@AdrianSkar/Basic-JS-iterate-for-loop).
+
+### Code explanation
+- Inititialization: `i` gets a value of `0` and its used as a counter.
+- Condition: the subsequent code is executed as long as `i` is lower than the length of `myArr` (which is 5; five numbers but arrays are zero based).
+- Final-expression: `i` is incremented by `1`.
+- Statement: The function adds `myArr[i]`'s value to `total` until the condition isn't met like so:
+
+```text
+total + myArr[0] -> 0 + 2 = 2 
+total + myArr[1] -> 2 + 3 = 5
+total + myArr[2] -> 5 + 4 = 9
+total + myArr[3] -> 9 + 5 = 14 
+total + myArr[4] -> 14 + 6 = 20
+```
+
+## Alternative code solution:
+
+```javascript
+for (var y = myArr.length - 1; y >= 0; y--) {
+  total += myArr[y];
+}
+```
+·  Run code at [repl.it](https://repl.it/@AdrianSkar/Basic-JS-iterate-for-loop).
+
+### Code explanation
+This works similarly to the last solution but it's faster<sup><a href="#cite1">1</a></sup> although it might not meet your requirements if order is important.
+- Initialization: `y` gets the `myArr.length`'s value once so the function doesn't need to check it at `condition` every time the loop is executed.
+- Condition: the loop is executed as long as `y` is greater than `0`.
+- Final-expression: `y` is decremented by `1`.
+- Statement: The function adds `myArr[y]`'s value to `total` until the condition isn't met like so:
+```text
+total + myArr[4] -> 0 + 6 = 6
+total + myArr[3] -> 6 + 5 = 11
+total + myArr[2] -> 11 + 4 = 15
+total + myArr[1] -> 15 + 3 = 18
+total + myArr[0] -> 18 + 2 = 20
+```
+### Sources
+<span id="cite1">1</span>. ["Are loops really faster in reverse?",  *stackoverflow.com*](https://stackoverflow.com/questions/1340589/are-loops-really-faster-in-reverse)
+
+### Resources
+
+- ["for" - *MDN JavaScript reference*](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for)
 
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