function Slider(config) {
  var slider = this;
  this.time = config.time || 3000;
  this.auto = config.auto || false;
  // this.slides = document.querySelectorAll(config.slides) || document.querySelectorAll(config.container + ' .slides img');
  this.slides = document.querySelectorAll('#slider1 .slides img');
  this.nextBtn = document.querySelector(config.nextBtn) || document.querySelector(config.container + ' .controls .next');
  this.prevBtn = document.querySelector(config.prevBtn) || document.querySelector(config.container + ' .controls .back');
  this.current = config.current || 0;

  if (slider.auto) {
    slider.run();
  }

  this.next = function() {
    slider.slides[slider.current].className = '';
    slider.current++;

    if (slider.current >= slider.slides.length) {
      slider.current = 0;
    }

    slider.slides[slider.current].className = 'visible';
  };

  this.prev = function() {
    slider.slides[slider.current].className = '';
    slider.current--;

    if (slider.current < 0) {
      slider.current = slider.slides.length - 1;
    }

    slider.slides[slider.current].className = 'visible';
  };

  this.run = function() {
    setInterval(function() {
      slider.next();
    }, slider.time);
  };

  slider.nextBtn.onclick = slider.next;
  slider.prevBtn.onclick = slider.prev;

}

new Slider({
    container: "#slider1",
    auto: true,
    time: 5000
  });

function Slider(config) {

  ...

  if (slider.auto) {
    slider.run();
  }
}