如何在字符串的指定位置插入新的字符

String.prototype.splice = function(start, newStr) {
    return this.slice(0, start) + newStr + this.slice(start);
};
var index = 'aaa_bbb'.indexOf('_'),
    str = 'aaa_bbb'.splice(index+1, 'ccc_');

function addStr(oldStr,addItem,afterWhich){
    var strArr = oldStr.split('');
    strArr.splice(oldStr.indexOf(afterWhich)+1,0,addItem);
    return strArr.join('');
}

//调用函数
addStr('aaa_bbb','ccc_','_')    //"aaa_ccc_bbb"