为什么这个方法不能在javascript中分配字符?

好的,下面是一个新手问题:

//function removes characters and spaces that are not numeric.

// time = "2010/09/20 16:37:32.37"
function unformatTime(time) 
{       

    var temp = "xxxxxxxxxxxxxxxx";

    temp[0] = time[0];
    temp[1] = time[1];
    temp[2] = time[2];
    temp[3] = time[3];
    temp[4] = time[5];
    temp[5] = time[6];
    temp[6] = time[8];
    temp[7] = time[9];
    temp[8] = time[11];
    temp[9] = time[12];
    temp[10] = time[14];
    temp[11] = time[15];
    temp[12] = time[17];
    temp[13] = time[18];
    temp[14] = time[20];
    temp[15] = time[21];   


}

在Firebug中,我可以看到有时的字符没有分配给temp? 在JS中,是否必须使用replace()函数来执行类似的操作?

谢谢您。