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JavaScript: Negation

Along with the conjunction (AND) and disjunction (OR) we often use the "negation" operator, too. It changes the logical value to the opposite one. It is denoted by the unary operator ! in programming.

If there is a function that checks for even numbers, then negation will allow you to check for odd numbers:

const isEven = (number) => number % 2 === 0;

isEven(10);  // true
!isEven(10); // false

I.e., we can just put a ! to the left of the function call and get the opposite result.

Negation is a powerful tool that allows you to concisely express the desired rules in your code without having to write new functions.

What if you wrote !!isEven(10) like this? Suddenly, the code would work. Double negation in logic is equivalent to no negation at all.

isEven(10);   // true
!isEven(10);  // false
!!isEven(10); // true


  1. Write a function, isPalindrome(), to check if a word is a palindrome. A palindrome is a word that reads the same backwards as it does forwards.

    isPalindrome('refer'); // true
    isPalindrome('level'); // true
    isPalindrome('hexlet'); // false
    // You can pass words to the function in any case
    // So first you have to make the word lower case: word.toLowerCase()
    isPalindrome('Madam'); // true

    To find a palindrome, you need to reverse the string and compare it to the initial one. To do this, use the reverse() function.

    reverse('hexlet'); // 'telhex'
  2. Write a function, isNotPalindrome(), to check if a word is NOT a palindrome:

    isNotPalindrome('level'); // false
    isNotPalindrome('wow'); // false
    isNotPalindrome('hexlet'); // true

    All you need is to negate the isPalindrome() function call within the isNotPalindrome() function's body.

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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