gah

packages/svelte/types/index.d.ts

@@ -625,7 +625,6 @@ declare module 'svelte/animate' {
 }
 
 declare module 'svelte/attachments' {
-	import type { Action, ActionReturn } from 'svelte/action';
 	/**
 	 * An [attachment](https://svelte.dev/docs/svelte/@attach) is a function that runs when an element is mounted
 	 * to the DOM, and optionally returns a function that is called when the element is later removed.
@@ -691,6 +690,75 @@ declare module 'svelte/attachments' {
 	 * ```
 	 * */
 	export function fromAction<E extends EventTarget>(action: Action<E, void> | ((element: E) => void | ActionReturn<void>)): Attachment<E>;
+	/**
+	 * Actions can return an object containing the two properties defined in this interface. Both are optional.
+	 * - update: An action can have a parameter. This method will be called whenever that parameter changes,
+	 *   immediately after Svelte has applied updates to the markup. `ActionReturn` and `ActionReturn<undefined>` both
+	 *   mean that the action accepts no parameters.
+	 * - destroy: Method that is called after the element is unmounted
+	 *
+	 * Additionally, you can specify which additional attributes and events the action enables on the applied element.
+	 * This applies to TypeScript typings only and has no effect at runtime.
+	 *
+	 * Example usage:
+	 * ```ts
+	 * interface Attributes {
+	 * 	newprop?: string;
+	 * 	'on:event': (e: CustomEvent<boolean>) => void;
+	 * }
+	 *
+	 * export function myAction(node: HTMLElement, parameter: Parameter): ActionReturn<Parameter, Attributes> {
+	 * 	// ...
+	 * 	return {
+	 * 		update: (updatedParameter) => {...},
+	 * 		destroy: () => {...}
+	 * 	};
+	 * }
+	 * ```
+	 */
+	interface ActionReturn<
+		Parameter = undefined,
+		Attributes extends Record<string, any> = Record<never, any>
+	> {
+		update?: (parameter: Parameter) => void;
+		destroy?: () => void;
+		/**
+		 * ### DO NOT USE THIS
+		 * This exists solely for type-checking and has no effect at runtime.
+		 * Set this through the `Attributes` generic instead.
+		 */
+		$$_attributes?: 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:
+	 * ```ts
+	 * export const myAction: Action<HTMLDivElement, { someProperty: boolean } | undefined> = (node, param = { someProperty: true }) => {
+	 *   // ...
+	 * }
+	 * ```
+	 * `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.
+	 */
+	interface Action<
+		Element = HTMLElement,
+		Parameter = undefined,
+		Attributes extends Record<string, any> = Record<never, any>
+	> {
+		<Node extends Element>(
+			...args: undefined extends Parameter
+				? [node: Node, parameter?: Parameter]
+				: [node: Node, parameter: Parameter]
+		): void | ActionReturn<Parameter, Attributes>;
+	}
+
+	// Implementation notes:
+	// - undefined extends X instead of X extends undefined makes this work better with both strict and nonstrict mode
 
 	export {};
 }