Question

I am trying to add text to each cell in particular column via Javascript. Like every 8th TD would be processed for adding text.

Tell me what I have done wrong here/why it doesn’t appear in my table:

<script type="text/javascript">
window.onload = function inventorytable() {
var tableRows = document.getElementById

("inventorytable").getElementsByTagName("tbody")[0].getElementsByTagName("tr");

for(var i = 0, l = tableRows.length; i < l; i++) {
    tds = tableRows.getElementsByTagName("td");
    var processor = tds[8].innerHTML += " Ghz"
    var ram = tds[9].innerHTML += " GB"
    var rspeed = tds[11].innerHTML += " Mhz"
}}
</script>

Answer 1

You’ve forgotten to refer to the i-th row:

tds = tableRows[i].getElementsByTagName("td");

I recommend to use:

window.onload = function inventorytable() {
    var tableRows = document.getElementById("inventorytable").rows;
             //or .tBodies[0].rows

    for(var i=0, l=tableRows.length; i < l; i++) {
        var tds = tableRows[i].cells;
        /*processor*/ tds[8].innerHTML += " Ghz"
        /*ram      */ tds[9].innerHTML += " GB"
        /*  rspeed */ tds[11].innerHTML += " Mhz"
    }
}

Do not define unused variables. If you want to clarify their use, use comments.

Also, I have replaced .getElementsByTagName by .rows and .cells.

Update
JavaScript sets are zero-based. So, if you want to refer to the 4th cell, use .cells[3].
At your previous answer, you’ve showed a fiddle. see http://jsfiddle.net/ndfh2/.

As you can see, the first row is also getting postfixes. To not add postfixes to these cells in the first row, initiate the counter at one: for( var i=1; .. ; .. )

Your current code does probably not work, because your rows don’t have twelve (12) rows. Remember, the number at tds[ number ] equals the index of a cell within a row, starting at zero.