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JavaScript: Returning from loops

Dealing with loops usually comes down to two cases:

  1. Aggregation. Accumulation of data during iterations and handling it after the loop. String reversal is just one of these cases
  2. Executing a loop until you get the required result and breaking from it. For example, if the job of the loop is to find prime numbers. Remember that a prime number is a number that can only be divided without a remainder by itself and by one

Consider a simple algorithm to check prime numbers. We will divide a given number x by all numbers between 2 and x - 1 and check the remainder. If we don't find a divisor that divides the number x without a remainder in this range, then we are looking at a prime number.

If you think about it, it's actually enough to check numbers not up to x - 1, but up to half the given number. For example, 11 is not divisible by 2, 3, 4, or 5. But it's also guaranteed that it can't be divided by numbers greater than its half. So, you can do a little optimization and check division only up to x / 2.

const isPrime = (number) => {
  if (number < 2) {
    return false;

  let divider = 2;

  while (divider <= number / 2) {
    if (number % divider === 0) {
      return false;

    divider += 1;

  return true;

isPrime(1); // false
isPrime(2); // true
isPrime(3); // true
isPrime(4); // false

The algorithm is built like so: if during the successive division by numbers up to x / 2, there is at least one result without a remainder, then the given argument is not a prime number, and therefore further computations are pointless. At this point, it returns false.

And only if the entire loop is completed can we say that the number is prime since no number by which it can be divided without a remainder can be found.


Write the hasChar() function that checks whether a string contains a given character (case sensitive). The function takes two arguments:

  • String
  • Character to look for
hasChar('Hexlet', 'H'); // true
hasChar('Hexlet', 'h'); // false
hasChar('Awesomeness', 'm'); // true
hasChar('Awesomeness', 'd'); // false
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 🙄

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