String.prototype.replace(str,func) 例中func的参数问题？

<div id="myDiv"></div>
  <script>
	String.prototype.supplant = function(o){
		return this.replace(/{([^{}]*)}/g,
				function (a,b){
					var r = o[b];
					return typeof r==='string'?r:a;
				}
		);
		
	};

	var template ='<table border="{border}">' +
	'<tr><th>Last</th><td>{last}</td></tr>' +
	'<tr><th>First</th><td>{first}</td></tr>'+
	'</table>';
	var data = {
		first:"Carl",
		last:"Hollywood",
		border:"2"
	};
	myDiv.innerHTML = template.supplant(data);
  </script>

function (a,b){
			var r = o[b];
			return typeof r==='string'?r:a;
		}