function Fn(str){

  this.str = str;

}

Fn.prototype.format = function(){

  var arg = arguments;

  var dd =  this.str.replace(/{[0-9]}/g,function(a,b){

    var _index = a.replace(/\{|}/g,'');

    console.log(_index);

    return arg[_index]||''

  })

  return dd;

}

var fn = new Fn('<p><a href="{0}">{1}</a></p>')

document.write (fn.format('http://www.baidu.com/','百度'));