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. Generally, you 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 many conveniences 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>;
  }
};

React's creators suggest resolving this issue using the classnames package, which is easy to use. The approach is straightforward: we generate an appropriate object instead of manipulating a string directly.

Then we convert this object 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
    // The class 'btn' will be substituted anyway
    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'

We can use the cn() function 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,
});

We can also set mandatory classes in the object:

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

Sometimes, we generate the class name dynamically. In that case, we can use the following code:

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)