Free JavaScript course. Sign Up for tracking progress →

JavaScript: Building strings with loops

Loops also allow you to generate strings. It's a common task for web development. It's merely basic aggregation using interpolation or concatenation.

One especially popular task among interviewers is to do a string reversal. There are lots of ways to solve it, but the most basic is by iterating each character. Have a look at how this function works:

reverse('Hexlet'); // telxeH

The general idea of the reversal is that you take characters one by one from the beginning of the string and then connect them in reverse order. Sounds like a piece of cake. Now let's examine it:

const reverse = (str) => {
  let i = 0;
  // A neutral element in terms of strings is an empty string
  let result = '';
  while (i < str.length) {
    // Connect it in reverse order
    result = `${str[i]}${result}`;
    // Same through concatenation
    // result = str[i] + result;
    i = i + 1;
  }

  return result;
};

const name = 'Bran';
reverse(name); // 'narB'
// Testing neutral element
reverse(''); // ''

Perhaps the only tricky part here is getting a feel of how the string itself is built. Since each next character is attached to the resulting string on the left, the string ends up reversed.

Instructions

Write the same reverse() function, except it doesn't iterate over a string from the first element to the last, but vice versa, from the last to the first. The overall structure of the function remains the same. The initial index, the loop termination condition, the building of a new string, and the formation of a new index in the loop will change, however.

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.

If you got stuck and don't know what to do, you can ask a question in our huge and friendly community