Classnames

classnames is a commonly used utility in modern frontend applications to conditionally join CSS class names together. If you've written React applications, you likely have used a similar library.

Implement the classnames function.

Examples

classNames('foo', 'bar'); // 'foo bar'
classNames('foo', { bar: true }); // 'foo bar'
classNames({ 'foo-bar': true }); // 'foo-bar'
classNames({ 'foo-bar': false }); // ''
classNames({ foo: true }, { bar: true }); // 'foo bar'
classNames({ foo: true, bar: true }); // 'foo bar'
classNames({ foo: true, bar: false, qux: true }); // 'foo qux'

Arrays will be recursively flattened as per the rules above.

classNames('a', ['b', { c: true, d: false }]); // 'a b c'

Values can be mixed.

classNames(
  'foo',
  {
    bar: true,
    duck: false,
  },
  'baz',
  { quux: true },
); // 'foo bar baz quux'

Falsy values are ignored.

classNames(null, false, 'bar', undefined, { baz: null }, ''); // 'bar'

In addition, the returned string should not have any leading or trailing whitespace.

Resources

classnames library on GitHub
clsx library on GitHub: A newer version that serves as a faster and smaller drop-in replacement for classnames.