Use Javascript for Calculation

Question

Hi I have One grid with few items and their quantities as follows
Item Id Item Name
--------------------
1           Pen
2           Book
3           Scale
------------------------------------------
Item  id Quantity Batch No  
--------------------------- ---------------
1             1   1          
2             8   1
3              2 1
1              8 2
2              5 2
from above table I want to find total quantity of particular item
For eg: Item Pan Has quantity 9
i want to do this using Java script
I am very new to Javascript Pls Help me.