Interface Action<Element, Parameter, Attributes>

Actions are functions that are called when an element is created. You can use this interface to type such actions. The following example defines an action that only works on <div> elements and optionally accepts a parameter which it has a default value for:

export const myAction: Action<HTMLDivElement, { someProperty: boolean } | undefined> = (node, param = { someProperty: true }) => {
  // ...
}
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Action<HTMLDivElement> and Action<HTMLDivElement, undefined> both signal that the action accepts no parameters.

You can return an object with methods update and destroy from the function and type which additional attributes and events it has. See interface ActionReturn for more details.

Docs: https://svelte.dev/docs/svelte-action