How do I send array to the server from JS and return a modified array and use it in JS

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Javascript

Python

I am new to programming as it is not my core field. Still somehow managed to create a website font end with core java. Now I am learning Flask as I want python at backend so that I could learn python and apply various modules of python in my website. I have some doubt. May be its too modest to ask here.:

How should I send array of numbers from Javascript to server that is using Flask.
Make some process on that array(I am using flask at backend. I have already made up a function that returns desired array. But that function is in JS and I don't wish to disclose the function on the client side as I will be refining that function over the time.).
Send the modified array back to the client-side.
Use that array in JS. (without using jQuery)

What I have tried:

I tried using JSON but it is getting more complicated as I just want to send numbers and return numbers.