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JavaScript: Conditions inside loops

Both the loop body and function body are places where statements are executed. So we can use everything we already learned inside it, such as conditionals.

Imagine a function that counts the number of times a letter appears in a sentence. Here's how it might work:

countChars('Fear cuts deeper than swords.', 'e'); // 4
// If there are no matches, the result is 0
countChars('Sansa', 'y'); // 0

Before examining its contents, answer these questions:

  • Is this an aggregate operation?
  • What will be the test condition for whether or not the character occurs?
const countChars = (str, char) => {
  let i = 0;
  let count = 0;
  while (i < str.length) {
    if (str[i] === char) {
      // Count only relevant characters
      count = count + 1;
    // Counter increases no matter what happens
    i = i + 1;

  return count;

This is an aggregate task. Although it doesn't count every character, you still have to parse through each character to calculate the sum.

A key feature of this loop is the condition inside its body. The variable count only increases when the current character matches the expected one.

Apart from that, it is a generic aggregate function that returns the number of required characters.


The function from the theory is case sensitive. It means 'A' and 'a' from its point of view are different characters. Write a version of this function that is not case sensitive:

countChars('HexlEt', 'e'); // 2
countChars('HexlEt', 'E'); // 2
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.

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