I saw this super simple, useful idea floating around in some projects and decided to implement the API myself instead of installing yet another "micro dependency." Plus, I can be more opinionated (assuming BEM, for example) when I use my own code.

classnames.js

export default function () {
  let className = ''
  let options = {}
  const classNames = []

  if (arguments.length > 1) {
    className = arguments[0]
    options = arguments[1]
    classNames.push(className)
  } else {
    options = arguments[0]
  }

  Object.entries(options).forEach(([key, value]) => {
    if (value) {
      if (['-', '_'].includes(key[0])) {
        classNames.push(className + key)
      } else {
        classNames.push(key)
      }
    }
  })

  return classNames.join(' ')
}

/* examples:

  classnames('foo', { 'bar': returnsTrue(), 'baz': someFalseyValue, 'qux': (() => 1)() })
  === 'foo bar qux'

  classnames('foo__bar', { '--baz': returnsTrue(), '--quz': someFalseyValue })
  === 'foo__bar foo__bar--baz'

  classnames({ 'foo--bar': true, 'foo--baz': someFalseyValue })
  === 'foo--bar'

*/