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Sim events

Table of contents

  1. Effects
  2. Events
  3. List of events

Effects

In Pokémon Showdown, as well as in its predecessor Pokémon Lab, everything in Pokémon is an effect.

There are 11 types of effects:

Effect type Target Examples
Species Pokémon Arceus, Silvally
Move Pokémon Grass Knot, Magnitude
Ability Pokémon Intimidate, Technician
Item Pokémon Leftovers, Choice Scarf
Status (non-volatile) Pokémon Sleep, Poison
Volatile status Pokémon Protect, Substitute
Slot condition Slot Healing Wish, Wish
Side condition Side Reflect, Tailwind
Terrain Field Grassy Terrain, Misty Terrain
Weather Field Rain Dance, Sunny Day
Pseudoweather Field Gravity, Trick Room

Effects which target a side also target every Pokémon on a side, and effects targetting the field also target every Pokémon in it.

For convenience, most effects have the same ID as the move, ability, or item that induces it. The only exceptions are effects with many moves associated with them, such as statuses, and certain volatile statuses (flinch, confusion, locked moves, and trapping).

NOTE: The target of an effect can be accessed as this.effectData.target. However, slot conditions don't support this feature yet.

Events

Nearly every effect has some sort of event listener, and each action in the battle fires an event. There are two main types of events: single events and global events. The key difference between them is that global events propagate, while single events don't.

The typical usage of an event is through onEvent handlers on effects which target the so-called event target. For example, the Item single event Primal is run on a Pokémon, and will invoke the onPrimal event handler of the held item —if there is one. As for global events run on a Pokémon, they will call the associated event handlers according to the event propagation mechanism, which shall be described later.

Event input

Pokémon Showdown event handlers have widely different event signatures. However, for the event system there are only two major differing signatures, which are the following.

onEvent(target, source, sourceEffect) {}
onEvent(relayVar, target, source, sourceEffect) {}

In these definitions, target and source correspond to the event target and event source respectively, and are not to be confused with target and source of a move or other actions. For instance, the events ModifyMove and TryHit work as follows:

Event name Event target Event source Source effect
ModifyMove Move user Move target Active move
TryHit Move target Move user Active move

The relayVar parameter is described in the Relay variables section.

NOTE: Moves passed to events (e.g. as sourceEffect) are always copies. These are called "Active Moves", and can be safely modified.

Event output

There are 3 different ways in which Pokémon Showdown may consume the return value of an event.

Consumed Description Examples
None The event is run for its side effects. AfterBoost, Faint, SwitchIn, Update
Cancellation token If the event is cancelled, the ongoing battle action is cancelled as well. DragOut, TryAddVolatile, TryHit, TryImmunity
Output The return value of the event is passed to other functions. The event is very likely to support a relay variable. BasePower, Damage, ModifyAtk, ModifyCritRatio

Event propagation

Event propagation is the mechanism by which multiple effects have their event handlers called when a global event is run. They are called in order, from higher to lower priority, as per their listed Priority values. The priority should be specified with the key `${handlerName}Priority`, where handlerName is the full name of the associated event handler.

While the event is being run, its propagation can be stopped by the cancellation mechanism.

Propagation may be vertical, horizontal, or of the sourceEffect type.

Vertical propagation

Since effects that target a side also target every Pokémon on it, a global event that fires on a Pokémon will also fire on that Pokémon's side. So, for instance, the global event TryHit, which is run on the target of a move, is also intercepted by the onTryHit handler of Mat Block, a side condition on the target's side. This phenomenon is event propagation in action.

Similarly, any event that fires on a side will also fire on the field. So, for instance, the global Effectiveness event can be captured by Delta Stream, a weather condition.

Overall, this mechanism follows the direction:

Pokémon → Slot → Side → Field

Horizontal propagation

Events which target a Pokémon also propagate from it to other active Pokémon on the field:

Propagates to Event handler pattern Event handler examples
Event source onSourceEvent onSourceModifyDamage, onSourceTryHeal
Foe Pokémon onFoeEvent onFoeBasePower, onFoeTryMove
Allied Pokémon onAllyEvent onAllyBoost, onAllyTryHitSide
Any Pokémon onAnyEvent onAnyBasePower, onAnySetWeather

sourceEffect propagation

This is an exceptional mechanism. The BasePower event also propagates to the sourceEffect of the event, i.e. the active move, and its event handler has maximum priority.

Event cancellation

A global event can be cancelled by any event handlers, which entails the following:

  • The execution of any pending event handlers will be aborted.
  • The global return value of the event will be set to the raised cancellation token.

Cancellation is triggered from any event handler by returning any of the cancellation tokens: this.FAIL, or this.SILENT_FAIL (where this is the active Battle).

NOTE: In legacy code, the cancellation tokens are hardcoded as false and null.

Priority events

Priority events are an special type of global events. These will treat any value other than undefined as a cancellation token. Therefore, only the first non-void event handler will be executed.

The only priority event currently defined is RedirectTarget.

Relay variables

The first parameter passed to 4-ary event handlers is given the name of "relay variable". It's important for events whose output is consumed by the simulator, because the return value of the event handlers updates the relay variable, so long as it's not void (i.e. undefined).

This means that if an event includes several handlers which multiply the relay variable by some factor each, the overall effect is the product of these factors. For instance, the effects of Bright Powder and Tangled Feet are accumulated.

"brightpowder": {
	onModifyAccuracy(accuracy) {
		if (typeof accuracy !== 'number') return;
		return accuracy * 0.9;
	},
	desc: "The accuracy of attacks against the holder is 0.9x.",
}
"tangledfeet": {
	shortDesc: "This Pokemon's evasiveness is doubled as long as it is confused.",
	onModifyAccuracy(accuracy, target) {
		if (typeof accuracy !== 'number') return;
		if (target && target.volatiles['confusion']) {
			return accuracy * 0.5;
		}
	},
}

For correctness, however, because of the way the console games handle truncation, it's often better to use the chainModify pattern, which automatically handles truncation.

The chainModify pattern

Battle#chainModify() is a void function which registers multipliers for the relay variable of an ongoing event.

For instance, here is the Technician ability:

"technician": {
	shortDesc: "This Pokemon's moves of 60 power or less have 1.5x power. Includes\
		Struggle.",
	onBasePowerPriority: 8,
	onBasePowerPriority(basePower, attacker, defender, move) {
		if (basePower <= 60) {
			return this.chainModify(1.5);
		}
	},
}

Now, let's work through an example of how chainModify() works:

In a Doubles Battle with Grassy Terrain active, and with an allied Battery Charjabug on the field, a Technician Roserade uses Grass Knot against a Dry Skin Helioptile.

To calculate the damage inflicted, the method Battle#getDamage() is invoked:

// Excerpt
let basePower = move.basePower;
if (move.basePowerCallback) {
	basePower = move.basePowerCallback.call(this, pokemon, target, move);
}
basePower = this.runEvent('BasePower', pokemon, target, move, basePower, true);

First, the basePowerCallback handler on the move is fired:

"grassknot": {
	basePowerCallback(pokemon, target) {
		let targetWeight = target.getWeight();
		if (targetWeight >= 200) return 120;
		if (targetWeight >= 100) return 100;
		if (targetWeight >= 50) return 80;
		if (targetWeight >= 25) return 60;
		if (targetWeight >= 10) return 40;
		return 20;
	},
}

Grass Knot's basePowerCallback function returns 20 as its base power against Helioptile, which weighs 6 kg.

Next, the battle fires the BasePower event, which is intercepted by the following handlers:

  • onBasePower(user, target) [on move]
  • onBasePower(user, target, basePower, move) [on user]
  • onAllyBasePower(user, target, basePower, move) [on allies]
  • onFoeBasePower(user, target, basePower, move) [on foe Pokémon]
  • onSourceBasePower(user, target, basePower, move) [on target]
  • onAnyBasePower(user, target, basePower, move) [on every Pokémon]

As mentioned above, events fired on a Pokémon also fire on the side and the field. Therefore, the events found are the following:

  • Roserade's Technician (User's onBasePower, priority 8)
  • Field's Grassy Terrain (Field's onBasePower, priority 0)
  • Charjabug's Battery (Ally's onAllyBasePower, priority 8)
  • Helioptile's Dry Skin (Foe's onFoeBasePower, priority 7)

These handlers are sorted by their listed priority.

  1. Technician's callback goes first, so it updates the base power modifier from its initial value of 1 to 1.5.
  2. Battery's handler is called so the base power modifier is updated to ~1.95.
  3. Dry Skin's handler is called. Since Grass Knot is not a Fire-type move, this step doesn't affect its base power.
  4. Grassy Terrain's handler is called. Since Grass Knot is a Grass-type move, and Roserade is grounded, the base power modifier is updated to ~3.80.
  5. After the BasePower event is run, we get a rounded final base power of 76.

NOTE: Since chainModify() is a void function, the relay variable isn't updated during steps 1-4 above.

List of events

Single events

NOTE: This list is incomplete. Refer to dev-tools/globals.ts for a full list, including function signatures, of the single events available on abilities, moves, items or statuses, corresponding to the AbilityEventMethods, MoveEventMethods, ItemEventMethods and PureEffectEventMethods interfaces.

Abilities and items

onStart(pokemon) [on ability]
onStart(pokemon) [on item]

Fired after a pokemon switches in, following the onSwitchIn event.

If two pokemon switch in at once (first turn, or after two pokemon faint simultaneously), both onStart events will fire after both pokemon have switched in, and the faster pokemon's onStart event will fire first.

Examples: [ability] Drizzle, [ability] Intimidate, [item] Air Balloon

Statuses (PureEffect)

durationCallback(pokemon, source, sourceEffect) [on status, on volatile]
durationCallback(pokemon, source, sourceEffect) [on slot condition]
durationCallback(side, source, sourceEffect) [on side condition]
durationCallback(field, source, sourceEffect) [on weather, on terrain, on pseudoweather]

Fired while calculating an effect's duration. Returns the duration in turns, including the current one.

source is the pokemon that inflicted the new effect, or null if no such pokemon did.

sourceEffect is the move or effect that started the new effect.

For instance, if Pikachu uses Toxic, source is Pikachu, and sourceEffect is Toxic as a move, but if you are poisoned by Toxic Spikes, source is null, and sourceEffect is Toxic Spikes as a side condition.

For instance, Rain Dance has a duration of 5 turns, but it increases to 8 turns if the user has Damp Rock as its item. Return 0 to have it never expire in BW.

NOTE: If the duration is constant, prefer using the duration property.

The effect's residual event is not fired when it expires - instead, its end event is fired when its residual event would otherwise have fired.

Do not use this for effects that don't expire during a residual. For instance, sleep expires through a sleep counter that decrements every beforeMove event, not every residual event.

Examples: [weather] Rain Dance, [side condition] Reflect

onStart(pokemon, source, sourceEffect) [on status, on volatile]
onStart(pokemon, source, sourceEffect) [on slot condition]
onStart(side, source, sourceEffect) [on side condition]
onStart(field, source, sourceEffect) [on weather, on terrain, on pseudoweather]

Fired when an effect starts, with the same function signature as durationCallback.

Useful for cancelling an effect that fails (e.g. Encore against a newly switched-in foe), for giving messages that an effect has started, and for initializing effect data (e.g. Substitute, Wish).

Examples: [volatile] Encore, [side condition] Reflect, [volatile] Substitute

onRestart(pokemon, source, sourceEffect) [on status, on volatile]
onRestart(pokemon, source, sourceEffect) [on slot condition]
onRestart(side, source, sourceEffect) [on side condition]
onRestart(field, source, sourceEffect) [on weather, on terrain, on pseudoweather]

Fired when trying to induce an effect that's already active, with the same function signature as durationCallback.

Useful for effects that can have multiple "layers".

Examples: [side condition] Toxic Spikes, [side condition] Pursuit, [volatile] Stockpile

onEnd(pokemon, source, sourceEffect) [on status, on volatile]
onEnd(pokemon, source, sourceEffect) [on slot condition]
onEnd(side, source, sourceEffect) [on side condition]
onEnd(field, source, sourceEffect) [on weather, on terrain, on pseudoweather]

Fired when an effect ends naturally, usually because its duration ends.

Most ways an effect can end are "natural". The biggest exception is when a pokemon swiches out. In general, if you would receive a message when the effect ends, the end event would be fired.

NOTE: NOT fired when a pokemon switches out (see the global event onSwitchOut).

NOTE: Setting the duration counter of the effect won't prevent the effect from ending.

Useful for giving messages that an effect has ended.

Examples: [volatile] Encore, [side condition] Reflect, [volatile] Substitute

Moves

NOTE: For an schematic breakdown of the hit steps, refer to simulator-doc.txt

beforeMoveCallback(user, target, move) [on move]

Fired before a pokemon uses a move it chose, but after the global BeforeMove event. Return true to prevent the move.

Moves not called directly (e.g. moves called by Assist, Metronome, or Sleep Talk) do not fire this event.

Examples: [move] Focus Punch

beforeTurnCallback(user) [on move]

Fired before a turn starts on every pokemon which will move.

Used for Focus Punch (for the "focusing" message) and Pursuit (to activate the side condition that listens for the foeSwitchOut event).

Examples: [move] Focus Punch, [move] Pursuit

onModifyMove(move, user, target) [on move]

Fired before a pokemon uses a move.

It is also fired for moves that bypass beforeMoveCallback (moves called by Assist, Metronome, or Sleep Talk).

NOTE: Base Power modifications should be handled in onBasePower, not in onModifyMove. This is because many moves have variable base power.

NOTE: Priority modifications should be handled in the global event onModifyPriority. By the time the move can be modified through onModifyMove, its position in the decision queue is already fixed.

Examples: [move] Secret Power, [move] Technoblast, [move] Weather Ball

onBasePower(basePower, attacker, defender, move) [on move]

Fired while calculating a move's base power. Return the modified base power.

Examples: [move] Facade, [move] Knock Off

onHit(target, user, move) [on move]

Fired when a move hits (doesn't miss). Return false to prevent the move's effects.

Also used for moves whose effects are too complicated to be represented in any other way.

Examples: [foe volatile] Protect, [move] Belly Drum, etc

Global events

NOTE: This list is incomplete. Refer to dev-tools/globals.ts for a full list, including function signatures, of the global events available, corresponding to the EventMethods interface.

Main loop events

onBeforeTurn(pokemon) [on pokemon]

Fired before a turn starts on every active pokemon.

Used for Gen 4 Custap Berry and the Gen 2 Locked Move volatile, to dynamically update the decision queue.

Examples: [item] Custap Berry (Gen 4), [volatile] Locked Move (Gen 2)

onSwitchIn(pokemon) [on pokemon]

Fired after a pokemon switches in.

If two pokemon switch in at once (first turn, or after two pokemon faint simultaneously), both onSwitchIn events will fire after both pokemon have switched in, and the faster pokemon's onSwitchIn event will fire first.

Examples: [side condition] Stealth Rock, [slot condition] Healing Wish

onBeforeSwitchOut(pokemon) [on pokemon]

Fired before a pokemon switches out.

Examples: [side condition] Pursuit

onSwitchOut(pokemon) [on pokemon]

Fired before a pokemon switches out, after the onBeforeSwitchOut event. Return true to interrupt and prevent the pokemon from switching out.

NOTE: DO NOT use it to implement trapping: set the trapping flag in the onFoeTrapPokemon event instead.

NOTE: NOTHING in USUM or earlier prevents a pokemon from switching out at this stage, so you most likely DO NOT want to ever return true.

Examples: [ability] Natural Cure, [ability] Regenerator

onModifyPriority(priority, user, target, move) [on user]

Fired when determining a move's priority. Return the move's priority.

Examples: [ability] Prankster

onResidual(pokemon) [on status, on volatile, on ability, on item]
onResidual(side) [on side condition]
onResidual(field) [on weather, on pseudoweather]

Fired at the end of each turn, but before fainted pokemon are switched in.

Useful for whatever end-of-turn effects this may have.

The duration counter of each effect is decremented automatically before this event is fired (see duration for details).

Examples: [volatile] Ghost-type Curse, [weather] Sandstorm

Isolated events

onType(pokemon) [on pokemon]

Fired when calculating the type of a Pokémon to override the default typing mechanics. Only used for Roost, Arceus, and Silvally.

Examples: [volatile] Roost, [pokemon] Arceus, [pokemon] Silvally

onImmunity(type, target) [on target]

Fired when determining whether or not a pokemon is immune to a move. Return false if it is immune.

Examples: [volatile] Magnet Rise

onImmunity(effectid, target) [on target]

Fired when determining whether or not a pokemon is immune to a non-move effect. Return false if it is immune.

Examples: [ability] Magma Armor, [ability] Overcoat, [item] Safety Goggles

Hit steps

NOTE: For an schematic breakdown, refer to simulator-doc.txt

onBeforeMove(user, target, move) [on user]

Fired before a pokemon uses a move it chose. Return false to prevent the move.

Mostly effects that prevent a user from moving, such as paralysis.

Moves not called directly (e.g. moves called by Metronome, Sleep Talk, or Pursuit) do not fire this event.

Examples: [volatile] Flinching, [status] fully paralyzed, [move] Focus Punch

onBasePower(basePower, attacker, defender, move) [on user]
onFoeBasePower(basePower, attacker, defender, move) [on foe Pokémon]
onAnyBasePower(basePower, attacker, defender, move) [on any Pokémon]

Fired while calculating a move's base power. Return the modified base power.

Examples: [ability] Dark Aura, [ability] Dry Skin, [ability] Technician

onModifyMove(user, target, move) [on user]
onFoeModifyMove(user, target, move) [on foe Pokémon]

Fired before a pokemon uses a move.

Examples: [ability] Adaptability, [ability] Infiltrator, [ability] Pixilate

onHit(target, source, move) [on target]
onSourceHit(target, source, move) [on source]

Fired when a move hits (doesn't miss).

Examples: [ability] Anger Point, [ability] Magician, [item] Enigma Berry

onDamage(damage, target, source, effect) [on target]

Fired while calculating damage, either from a move or from a condition. Return a damage value directly in HP. Bypasses weaknesses and resistances, but not immunities.

Examples: [ability] Magic Guard, [volatile] Endure