IE下javascript获取textarea光标的开始结束位置

项目开发中需要计算textarea光标的开始位置与结束位置，以便于在指定位置插入文本。下面这段代码测试通过，能够得到光标的开始位置与结束位置。var er = document.selection.createRange(),start = 0,end = 0;

					if(er.parentElement().id === this.id){
						// create a selection of the whole textarea
						var er1 = document.body.createTextRange();
						er1.moveToElementText(this);
						//两个range，一个是已经选择的text(er)，一个是整个textarea(er1)
						//er1.compareEndPoints()比较两个端点，如果er1比er更往左(further to the left)，则 //返回小于0的值，则er1往右移一点，直到两个er的start相同。
						// calculate selection start point by moving beginning of er1 to beginning of er
						for (start = 0;er1.compareEndPoints('StartToStart',er) < 0;start++){
							er1.moveStart('character', 1);
						}
						// get number of line breaks from textarea start to selection start and add them to start
						// 计算一下\n
						for (var i = 0; i <= start;i++){
							if (this.value.charAt(i) == '\n'){
								start++;
							}
						}
						// create a selection of the whole textarea
						var er1 = document.body.createTextRange();
						er1.moveToElementText(this);
						// calculate selection end point by moving beginning of er1 to end of er
						for (end = 0;er1.compareEndPoints('StartToEnd',er) < 0;end++){
							er1.moveStart('character',1);
						}
						// get number of line breaks from textarea start to selection end and add them to end
						for (var i = 0;i <= end;i++){
							if (this.value.charAt(i) == '\n'){
								end ++;
							}
						}
					}