写一个原型继承的例子

function Elem(id) {
	this.elem = document.getElementById(id);
}
Elem.prototype.html = function(val){
	var elem = this.elem;
	if(val) {
		elem.innerHTML = val;
		return this;
	}else {
		return elem.innerHTML;
	}
}
Elem.prototype.on = function(type,fn){
	var elem = this.elem;
	elem.addEventListener(type,fn);
}

var div1 = new Elem('div');
console.log(div1.html());

div1.html('<p>假装自己是一段文字</p>').on('click', function(){
		alert('clicked')
	}
);