js返回字符串中某個字符的出現次數

　　使用此函數能返回字符串中某個字符或字符串出現的次數其中mainStr為要查找的字符串subStr為要檢查的字符串或字符

　　function countInstances(mainStr subStr)
    {
        var count = ;
        var offset = ;
        do
        {
            offset = mainStrindexOf(subStr offset);
            if(offset != )
            {
                count++;
                offset += subStrlength;
            }
        }while(offset != )
        return count;
    }

From:http://tw.wingwit.com/Article/program/net/201311/14350.html