Fast hasClass function.

hasclass.js

/**
 * Detect is an element contains a class name.
 *
 * @param   {Element} el - HTML element
 * @param   {string} name - Class name
 * @returns {boolean} `true` if the element contains the class name.
 */
function hasClass(el, name) {
  let classes = name && el && el.className
  if (classes) {
    if (typeof s != 'string') {
      s = el.getAttribute('class') || ''  // may be SVG
    }
    for (let n = 0; ~(n = classes.indexOf(name, n)); n++) {
      if (!n || /\s/.test(classes[n - 1])) {
        n += name.length
        if (n === classes.length || /\s/.test(classes[n])) {
          return true
        }
      }
    }
  }
  return false
}

See: https://jsperf.com/hasclass-function/1