function Animation(elem){
// code tests the application of js arrow function
// date August 18, 2018
this.elemid = document.querySelector('#'+elem)
}
Animation.prototype = {
constructor: Animation,
color: ['red', 'blue', 'black', 'yellow'],
display:function(){
var len = this.color.length;
var index = 0;
setInterval(()=>{
if(index < len){this.elemid.style.backgroundColor = this.color[index];
index++;
}else{
index =0;
this.elemid.style.backgroundColor = this.color[index]
}

},3000);
}
};
	var testDiv = new Animation('test');// create instance of the class
	testDiv.display();
	var test2 = new Animation('test2');// create another instance
	test2.display()