Callbacks (C→JS events)

The other direction: functions the game core calls at gameplay events. There are 15 GameHook_* callbacks. Three of them surface to the renderer via EM_ASM → a window.__on* callback; the rest are C→C, meaning engine-internal accessors and the wrappers invoked by the Wasm* exports.

Sources: haptic_events.c, game_hooks.c, check_triggers.c, item_overrides.c, cheats.c, transition_events.c

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JS-facing events (register window.__on*)

The renderer installs these globals; the core calls them when something happens.

window.__onItemReceived(itemId, method)

Fired by GameHook_NotifyItemReceived(item_id, method) whenever Link receives an item (chest, NPC, drop). method distinguishes receipt styles. Drives the inventory/checks trackers.

window.__onHapticEvent(type, param)

Fired by the eight haptic hooks in haptic_events.c. type is the event enum below; param carries intensity data (damage, item id, swing type) or 0. The renderer maps these to controller rumble, described in Haptics. type has to match HapticEventType in the JS haptics module.

type	Hook	param
0	GameHook_NotifySwordSwing	swing type
1	GameHook_NotifySwordHitEnemy	damage dealt
2	GameHook_NotifySwordClink	0
3	GameHook_NotifyDamageTaken	damage amount
4	GameHook_NotifyItemUsed	item id
5	GameHook_NotifyEnvironmentalEvent	event subtype (fall, landing, bomb, water, mirror, quake, boss, and so on)
6	GameHook_NotifyHookshotWall	0
7	GameHook_NotifyBoomerangCatch	0

window.__onTransitionSettled(module, fromSubmodule, isIndoors, roomIndex, owScreenIndex)

Fired by GameHook_ModuleFrameEnd, called once per frame from Module_MainRouting. Reports the frame the game hands control back to the player: main_module_index enters a gameplay module (7 dungeon, 9 overworld), or submodule_index inside one returns to 0. That is the frame on which a room transition, door animation, shutter close or stair climb has finished and the room's collision is final, for every dungeon submodule at once, since the dungeon module's own dispatch treats submodule 0 as settled. fromSubmodule is 0 when this is a module-enter edge, otherwise the submodule that just finished. See Transition Events for the classification and the renderer subscription.

Gated on the Developer Tools setting (off by default): the hook makes zero host-calls, and GameHook_ModuleFrameEnd costs nothing beyond the flag check, when it's off.

C→C callbacks (no JS event)

These are called from the vendored game code or from the Wasm* wrappers. They're useful to know when tracing behavior, but you don't register anything for them.

Callback	Called from	Purpose
GameHook_OverrideChestItem(room, slot, orig)	chest-open in player.c	Returns the randomizer replacement item for the chest slot (see Item Overrides).
GameHook_GetDamageMultiplier()	sprite.c damage calc	Returns the cheat outgoing-damage multiplier.
GameHook_GetExtraArmorPct()	player.c damage reduction	Returns the cheat extra-armor %.
GameHook_TriggerCheck(room, chest, item)	WasmTriggerCheck wrapper	Marks a chest check + delivers the item.
GameHook_TriggerNpcCheck(...)	WasmTriggerNpcCheck wrapper	Sets a progress flag, advances the NPC, delivers the item.

Adding a new event

A new C→JS event mirrors an export's two-place rule in reverse: add the GameHook_* function (declared in game_hooks.h), EM_ASM a window.__on* call, insert the call-site in the vendored core at the right gameplay event, and register the handler in the renderer. See the add-wasm-function skill and Adding a WASM Function.