Update table according with JSON/JSONB

I have a table books :

And a table book_modifications :

Now let's imagine I have a row in my books table like this :

id: 15,
name: "name-1",
nb_pages: 120

And a row to insert in my book_modifications table like this :

id: 7,
old_data: {name: "name-1", nb_pages: 120},
new_data: {name: "new-name-1"},
book_id: 15

My question is :

How can I update my books table with the new_data column of my book_modifications table in SQL or PostgreSQL ?

Currently I have found this way :

create or replace function on_book_modifications_insert () 
returns trigger 
language plpgsql 
as $$
begin

-- Here (new_data only contains the 'name' field)
UPDATE books 
SET name = new.new_data -> 'name', nb_pages = new.new_data -> 'nb_pages' 
WHERE id = new.book_id;

return new;

end;
$$

In this case, the SQL query above will update nb_pages to null because it is not present in new_data (which will generate an error), whereas I would like to leave nb_pages at 120 and only update fields present in new_data.

For INSERT, I've seen that json_object exists, but I can't find it for UPDATE.

You can just use COALESCE against the original value. If the JSONB value doesn't exist then you get null, which is replaced with the original, so effectively a no-op.

create or replace function on_book_modifications () 
returns trigger 
language plpgsql 
as $$
begin

-- Here (new_data only contains the 'name' field)
UPDATE books
SET
  name = COALESCE(new.new_data ->> 'name', books.name),
  nb_pages = COALESCE((new.new_data ->> 'nb_pages')::int, books.nb_pages)
WHERE books.id = new.book_id;

return new;

end;
$$

db<>fiddle