in eosjs 20.x and 21.x, get_table_rows uses table_key argument, and it's described in the documentation as secondary key name, presumably usable instead of index_position.

In fact, chain_plugin does not recognize table_key, so the examples don't work, as you can see in several bug reports.

Either table_key needs to be removed from eosjs completely, or eosjs needs an additional lookup to get the index names and then pass the corresponding index_position in get_table_rows.