Create multi upsert query

[Mkbewe]

i want to create method to do upsert using library pg-format.

Example, the id is serial field.

// Example data to save:
const data = [
    {id: null, name:'a', desc: 'b'},
    {id: 20, name: 'aa', desc: 'bb'}
]

const sql = 'INSERT INTO table_name(id, a, b)
    VALUES(%L)
    ON CONFLICT (id)
    DO UPDATE SET (a,b) = 
    (EXCLUDED.a, EXCLUDED.b)';

const params = []
for (obj of data) {
    const id = obj.id ?? 'DEFAULT';
    params.push([id, obj.name, obj.desc])
}

format(sql, params); 
// Result: 
//  INSERT INTO table_name(id, a, b)
//  VALUES('DEFAULT', 'a', b), ('20', 'aa', 'bb')
//  ON CONFLICT (id)
//  DO UPDATE SET (a,b) = 
//  (EXCLUDED.a, EXCLUDED.b)

So I got an error because "DEFAULT" is in quotation marks, so it is a string and not the keyword postgres. I try using %I but it also not work. The only way i found was to use %s but it is not esceped so it's unsave.
I also try to make combination of this marks like this VALUES(%s, %L) or VALUES(%I, %L) but i i can have both id as a number and as a null so it's a incorrect way.

Mayby is diffret way to create upsert query or to bind this?