Something short and sweet: let's explore one of the better ways to write conditional classes in React. I will use a simple menu item functional component and some pseudocode.

const MenuItem = ({ children }) => {
  // TODO: add classes when a menu item is "active"
  // TODO: optionally add classes for dark theme
  return (
    <li className="menu-item">
      {children}
    </li>
  );
};

First attempt: Logical && operator

const MenuItem = ({ children }) => {
  return (
    <li className={`menu-item ${isActive && 'menu-item-active'}`}>
      {children}
    </li>
  );
};

Cons: This code actually has a hidden bug. For falsy values the false css class appears in the DOM. Let's get rid of the bug in our next attempt.

Second attempt: Ternary operator

const MenuItem = ({ children }) => {
  return (
    <li className={`menu-item ${isActive ? 'menu-item-active' : ''}`}>
      {children}
    </li>
  );
};

Cons: This code does not have any hidden bugs like before, but looks too complex for such a simple task. It is also not so readable as it should be and can get event worse: imagine having other use conditions like isDarkTheme.

Third attempt: Constructing className strings

The third attempt and my personal favorite is using a function (npm package) to construct the className. It is actually a mix of the first 2 attempts:

const MenuItem = ({ href, children }) => {
  return (
    <li className={classnames(
      'menu-item',
      isActive && 'menu-item-active',
      isDarkTheme && 'menu-item-dark'
    )}>
      {children}
    </li>
  );
};

NPM packages

Here is a list of NPM packages and their bundle sizes. They all have the same implementations, so you can choose which one suits you best:

• classnames with a tiny 729 bytes
• clsx with a tiny 516 bytes
• merge-class-names at an incredibly tiny 372 bytes

Conclusion

We as developers always read more code than we write. That's why I always choose to write code that is more readable and easily understandable by anyone.