JavaScript: For loop
A while loop works perfectly for situations where the number of iterations isn't known in advance, e.g., finding a prime number. When we know the number of iterations, it's better to use a for loop.
Let's take a look at a function that reverses strings using a for loop:
const reverseString = (str) => {
let result = '';
for (let i = 0; i < str.length; i += 1) {
result = `${str[i]}${result}`;
}
return result;
};
We could read it as follows: a loop with index i repeats until i < str.length and after each step increases i by 1.
When defining the for loop, we have three expressions in parentheses, separated by semicolons:
- The initial counter value. This code runs exactly once before the first iteration.
- A predicate - the condition for the loop to iterate. It's executed on each iteration. Exactly like what we have with
while. - Description of the counter change. This code is executed at the end of each iteration.
Other than that, it works exactly the same as the while loop.
Instructions
Samwell discovered that his messages were being intercepted at Castle Gemini and being read there. This made his attacks no longer a surprise. After some thought, he developed a program to encrypt the messages according to the following algorithm. It would take the text and rearrange it every two consecutive characters.
encrypt('move'); // 'omev'
encrypt('attack'); // 'taatkc'
// If the number of characters is odd
// the last character remains unchanged
encrypt('go!'); // 'og!'
Write a function, encrypt(), which takes the original message and returns an encrypted one.
Think about it. Can this function decrypt an encrypted message?
The exercise doesn't pass checking. What to do? 😶
If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:
- Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨
Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.
My code is different from the teacher's one 🤔
It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.
In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.
I've read the lessons but nothing is clear 🙄
It's hard to make educational materials that will suit everyone. We do our best but there is always something to improve. If you see a material that is not clear to you, describe the problem in “Discussions”. It will be great if you'll write unclear points in the question form. Usually, we need a few days for corrections.
By the way, you can participate in courses improvement. There is a link below to the lessons course code which you can edit right in your browser.
Your exercise will be checked with these tests:
1import { expect, test } from 'vitest';
2import f from './index.js';
3
4test('test', () => {
5 expect(f('attack')).toEqual('taatkc');
6 expect(f('go!')).toEqual('og!');
7 expect(f('back!')).toEqual('abkc!');
8});
9Teacher's solution will be available in:
