SOURCE

//**************
function bar(name) {
  this.instance = null;
  this.name = name
};
bar.prototype.getName = function() {
  return this.name
};
bar.getInstance = function(name) {
  if (!this.instance) {
    this.instance = new bar(name)
  }
  return this.instance
};
var a = bar.getInstance('a');
var b = bar.getInstance('b');
// alert(a === b);
//**************
function once(fn) {
  var called = false
  return function() {
    if (!called) {
			called = true
      fn.apply(this, arguments)
    }
  }
}

function createModal(){
	var div = document.createElement('div');
	div.innerHTML = '弹窗';
	document.body.appendChild(div);
	return div
}
var modal = once(createModal)
modal()