输入框获取光标焦点并添加数据

//获取光标

function doGetCaretPosition (oField) {
            // Initialize
            var iCaretPos ;

            // IE Support
            if (document.selection) {

                // Set focus on the element
                oField.focus();

                // To get cursor position, get empty selection range
                var oSel = document.selection.createRange();

                // Move selection start to 0 position
                oSel.moveStart('character', -oField.value.length);

                // The caret position is selection length
                iCaretPos = oSel.text.length;
            }

            // Firefox support
            else if (oField.selectionStart || oField.selectionStart == '0')
                iCaretPos = oField.selectionStart;
            console.log(iCaretPos)
            // Return results
            return iCaretPos;
        }

//截取字符串

String.prototype.insert = function (index, item) {
            var temp = [];
            for (var i = 0; i < this.length; i++) {
                temp.push(this[i]);
            }
            temp.splice(index, 0, item);
            console.log(temp.join(""))
            return temp.join("")
        };

//js

string.insert(0,'jia')