For infantry archers, with either longbow or crossbow, the bow is generally their primary weapon. A longbow/crossbow isn't something you can easily wear at your side, so it's difficult to have with you as a sidearm in battle. The typical sidearms for Medieval European archers would be,

• According to literature, mostly swords and axes, and sometimes other weapons, such as mallets. From various accounts of the Battle of Agincourt, "having axes or swords hanging from their belts", "swords, hatchets, mallets, axes, falcon beaks and other weapons" (from the accounts collected and compared in Anne Curry, The Battle of Agincourt: Sources and Interpretations, Boydell Press, 2000, pg 160 & 162). Archers would also use weapons other than their usual sidearms, such as the large mallets used to drive in stakes for field fortification.

• According to artwork, mostly swords: example of longbowmen with swords and example of crossbowmen with swords. Artwork doesn't always show sidearms, so these well-armed (and often armoured) Late Medieval archers might not be representative of earlier Medieval archers. In same case, a large knife is shown as the sidearm.

Archers would use their sidearms if enemy cavalry or infantry successfully closed with them, or if they could do so more usefully than by shooting (which could easily be the case if they ran out of arrows) and had discipline and orders (or sufficient eagerness) to do so. Ideally, it isn't necessary for archers to engage in close combat. How often did it happen? In some battles, it happened (if due to enemy cavalry successfully closes, it would often be a lost battle), and in some, it didn't happen. Major battles were rare, and for many archers who fought in one, it would be the only major battle they ever fought in.

For East Asian infantry archers, the situation is largely the same. From Japanese battle tales, an infantry archer would use his bow until he had shot all of his arrows, and would then use other weapons in close combat.

For cavalry archers (mounted archers), the bow might either be their primary weapon, with other weapons used at need, or it might be one among a number of major weapons, which could be used as the situation demanded. For the latter cases, the bow might not even be the most likely to be first used (notably, in the case of cavalry armed with both muskets and bows (as these Tibetan cavalry are)). It was not unusual for armoured cavalry archers, often equipped with lance, sword, shield, mace, and bow, to fight in close combat. They would expect to do so often, possibly after using their bows earlier in the battle.