The simplest vanilla js plugin template

plugin.js

// <div id="box">Lorem</div>

// Our goal is to make text color red
var Makered = function(selector) {
  this.el = document.querySelector(selector); // for multiple elements use querySelectorAll
}

Makered.prototype.makered=function() {
  this.el.style.color="red";
}

var box = new Makered('#box');
box.makered();


// === WITH DEFAULT OPTIONS === //
//http://stackoverflow.com/a/42703312/4991434

//<p class="text">Lorem</p>

var Xstyle = function(elem, options) {
  this.el = document.querySelector(elem); // for multiple elements use querySelectorAll
  this.options = options || {};

  for (var optionName in this.defaultOptions) {
    if (typeof this.options[optionName] === 'undefined') {
      this.options[optionName] = this.defaultOptions[optionName];
    }
  }

}

Xstyle.prototype.defaultOptions = {
  color: "#222",
  backgroundColor: "transparent",
  fontSize: "16px"
}

Xstyle.prototype.xstyle = function() {
  this.el.style.backgroundColor = this.defaultOptions.backgroundColor;

  if (typeof this.options.backgroundColor !== 'undefined') {
    this.el.style.backgroundColor = this.options.backgroundColor;
  }
  if (typeof this.options.color !== 'undefined') {
    this.el.style.color = this.options.color;
  }
  if (typeof this.options.fontSize !== 'undefined') {
    this.el.style.fontSize = this.options.fontSize + "px";
  }

}

/* = Call plugin = */

var myText = new Xstyle(".text", {
  backgroundColor: "#ccc",
  color: "#444",
  fontSize: 20
});

myText.xstyle();