Typescript version of the classNames util w/ es6 export, so typescript is able to import it without confusion.

classNames.ts

// Based on: https://github.com/JedWatson/classnames/blob/master/index.js

/*!
  Copyright (c) 2018 Jed Watson.
  Licensed under the MIT License (MIT), see
  http://jedwatson.github.io/classnames
*/
var hasOwn = {}.hasOwnProperty;

export function classNames(...args: any[]) {
  var classes = [];

  for (var i = 0; i < args.length; i++) {
    var arg = args[i];
    if (!arg) continue;

    var argType = typeof arg;

    if (argType === 'string' || argType === 'number') {
      classes.push(arg);
    } else if (Array.isArray(arg) && arg.length) {
      var inner = classNames.apply(null, arg);
      if (inner) {
        classes.push(inner);
      }
    } else if (argType === 'object') {
      for (var key in arg) {
        if (hasOwn.call(arg, key) && arg[key]) {
          classes.push(key);
        }
      }
    }
  }

  return classes.join(' ');
}