Computed fields in web forms

Hi community,
I’m working on a web form to help my customers define their project requirements.
I added data fields in a parent form, and a table field that I populated in a child form.
The table includes 6 columns where 4 of them are numbers.
I need to calculate the sum of the values of both the lines and the columns of the table, and then display this number in the parent form.
I know some javascipt, but I never used it with erpnext, and I don’t know how to start.
So thanks for helping !