Simple class names concatenation in ES6 for React and others

classNames.js

const classNames = (...args) => args.reduce((list, clss) => list.concat(clss).filter(Boolean), [])
  .join(' ').trim().replace(/\s{2,}/g, ' ');

const test = null;
const test2 = true;

console.log(classNames(test && 'test1'));
console.log(classNames(test2 && 'test2'));
console.log(classNames(['test1', 'test2', test && 'test3', test2 && 'test4'], 'test5'));