Question about child themes

Hello, great plugin here.
I have a question before i use this plugin. i have recently learn what a child theme is and i have read some tutorials but im still not convinced that this is what i need. Let me explain myself.
In my website, i have modify many files of the theme. i Haven’t just add more functions on the functions.php or style.css i have add code in some javascript files.

My question is this: if a new theme update has change for example the file custom.js, this update will not appear on my child theme setup because in my child theme i have my version of custom.js am i right? i must somehow append something to the parent’s theme custom.js. But what if the append isn’t possible
What if my modification is on an already defined function? Should i alter this function manually on the new theme version to match my version?

So, to sum up, Child themes are good if your modifications can work on an append-style way? and you just for example what to include more styles on the .css or more functions on the functions.php? Doesnt child themes help much on altering already defined code?
I hope i am clear, thanks

https://wordpress.org/plugins/child-themify/