want to concatenate the all the line item fields in one line item field using scripting

Hello everyone, 

I have a requirement where i have table with multiple line item and i want to concatenate the all the subsequent line items in tabel from same column into first line item field, For example after executing the form if there are three line item lets say

100

200 

300

then first line item field should have 100,200,300 stored in it for all the line items that will be ther after executing the form using scripting 

I have tried using FOR loop but it keeps repeating way too many times 

Any help will be appriciated