Handling class names — JS: React

Interactive UI elements have more than one display state. For example, the modal window may be open or closed, and the switch may be on or off. It's generally accepted that you should change these states using classes.

When you work directly with the DOM, you can use classList, which contains convenient methods for adding and removing classes. React doesn't offer much convenience out of the box. The className property is just a string, and strings are awkward to process.

class Button extends React.Component {
  render () {
    const { isPressed, isHovered, label } = this.props;
    let btnClass = 'btn';
    if (isPressed) {
      // We have to concatenate classes
      btnClass += ' btn-pressed';
    } else if (isHovered) {
      btnClass += ' btn-over';
    }
    return <button className={btnClass}>{label}</button>;
  }
};

To solve this problem, the creators of React recommend using the classnames package. Its principle is simple: instead of manipulating a string directly, you need to generate the right object, which will then be converted into a string.

import cn from 'classnames';

class Button extends React.Component {
  render () {
    const { isPressed, isHovered, label } = this.props;
    // the value is true or false. If true, the class will be enabled, if false, it won't be
    // 'btn' is the class that will be substituted in any case
    const btnClass = cn('btn', {
      'btn-pressed': isPressed,
      'btn-over': !isPressed && isHovered,
    });
    return <button className={btnClass}>{label}</button>;
  }
};

Let's substitute specific values:

const btnClass = cn('btn', {
  'btn-pressed': false,
  'btn-over': true,
});

console.log(btnClass); // 'btn btn-over'

The cn() function is designed to accept any number of arguments as input. If the argument is a string, it's considered a required class. If it's an object, then the logic described above will work.

const btnClass = cn('btn', 'another-class', {
  'btn-pressed': isPressed,
  'btn-over': !isPressed && isHovered,
});

Mandatory classes can also be set in the object itself:

const btnClass = cn({
  'btn something-else': true
  'btn-pressed': isPressed,
  'btn-over': !isPressed && isHovered,
});

Sometimes, the class name is generated dynamically. In that case, the following code can be used:

const buttonType = 'primary';
const btnClass = cn('btn', `btn-${buttonType}`);
console.log(btnClass); // 'btn btn-primary'
// Or what's the same
// const btnClass = cn('btn', {
//   [`btn-${buttonType}`]: true
// });

Recommended materials

1. classnames (documentation)

Are there any more questions? Ask them in the Discussion section.

The Hexlet support team or other students will answer you.