code: function(el, config) {
  
    config = config || {};
  
    if (!el) {
      throw new Error("You must supply either a HTMLSelectElement or a CSS3 selector string.");
    }
  
    this.el = el;
  
    // CSS3 selector string
    if (typeof el === "string") {
      this.el = document.querySelector(el);
    }
  
    if (this.el === null) {
      throw new Error("The element you passed to Selectr can not be found.");
    }
  
    if (this.el.nodeName.toLowerCase() !== "select") {
      throw new Error("The element you passed to Selectr is not a HTMLSelectElement.");
    }
  
    this.render(config);
  }
docs: Main Lib