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 = 0;
        var offset = 0;
        do
        {
            offset = mainStr.indexOf(subStr, offset);
            if(offset != -1)
            {
                count++;
                offset += subStr.length;
            }
        }while(offset != -1)
        return count;
    }