Debounce JS function with leading / fire-first option

debounce.js

function debounce(fn, delay, leading) {
  var timeout
  var firedAt
  return function() {
    var args = arguments
    function fire () {
      firedAt = Date.now()
      fn.apply(null, args)
    }
    if (timeout) clearTimeout(timeout)

    if (leading === true) {
      var nextFireAt = delay - (Date.now() - firedAt)
      if (!firedAt || nextFireAt <= 0) {
        fire()
      } else {
        timeout = setTimeout(fire, nextFireAt)
      }
    } else {
      timeout = setTimeout(fire, delay)
    }
  }
}