js判断字符串出现的次数

function countSubstr(str, substr, isIgnore) {
    var count;
    var reg = "";
    if (isIgnore == true) {
        reg = "/" + substr + "/gi";
    } else {
        reg = "/" + substr + "/g";
    }
    reg = eval(reg);
    if (str.match(reg) == null) {
        count = 0;
    } else {
        count = str.match(reg).length;
    }
    return count;
}