向原型方法添加回调函数并设置此上下文

function employee(name, jobtitle, born) {
    this.name=name;
    this.jobtitle=jobtitle;
    this.born=born;
    this.callback = function(){};
}

employee.prototype.output = function(target){
    var whenHidden = function () {
        target.textContent = this.name + " is a " + this.jobtitle + 
                             " born in the year " + this.born;
        target.removeEventListener('transitionend', whenHidden);
        target.classList.remove('hide');
    }.bind(this);
    target.addEventListener('transitionend', whenHidden);
    target.classList.add('hide');
};

var fred = new employee("Fred Flintstone", "Caveman", 1970);
var mike = new employee("Mike Francois", "Wrestler", 1965);
var demo = document.getElementById("demo");
var output = employee.prototype.output;

document.getElementById("a").addEventListener('click', output.bind(fred, demo));
document.getElementById("b").addEventListener('click', output.bind(mike, demo));

p { 
  border:2px black solid;
  transition:.5s;
  height:30px;
  text-overflow: clip;
  overflow: hidden;
  opacity: 1;
}
p.hide {
  height: 1px;
  opacity: 0.1;
}

<button id="a">Fred</button>
<button id="b">Mike</button>
<button id="c">Tom</button>
<p id="demo"></p>