Storing number inside a string variable

programming flash
1

For my calculator i am having trouble storing a number inside a string variable.

Calculator, Layer ‘actionscript’, Frame 1, Line 144 1061: Call to a possibly undefined method append through a reference with static type String.

var currentNumber:String ="";
function pressNumber(e:MouseEvent):void{
	       display_txt.appendText(e.target.num);
		   currentNumber.append(e.target.num);//error here
}

How can i resolve this??