[feat] TS interfaces for typing actions (sveltejs#7121)

Fixes sveltejs#6538

.gitignore

@@ -9,6 +9,7 @@ node_modules
 /compiler.*js
 /index.*js
 /ssr.*js
+/action
 /internal
 /store
 /easing

package.json

@@ -39,6 +39,9 @@
       "import": "./compiler.mjs",
       "require": "./compiler.js"
     },
+    "./action": {
+      "types": "./types/runtime/action/index.d.ts"
+    },
     "./animate": {
       "types": "./types/runtime/animate/index.d.ts",
       "import": "./animate/index.mjs",

src/runtime/action/index.ts

@@ -0,0 +1,42 @@
+/**
+ * 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.
+ * - destroy: Method that is called after the element is unmounted
+ * 
+ * Example usage:
+ * ```ts
+ * export function myAction(node: HTMLElement, paramater: Parameter): ActionReturn<Parameter> {
+ *   // ...
+ *   return {
+ *     update: (updatedParameter) => {...},
+ *     destroy: () => {...}
+ *   };
+ * }
+ * ```
+ * 
+ * Docs: https://svelte.dev/docs#template-syntax-element-directives-use-action
+ */
+export interface ActionReturn<Parameter = any> {
+	update?: (parameter: Parameter) => void;
+	destroy?: () => void;
+}
+
+/**
+ * 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 }> = (node, param = { someProperty: true }) => {
+ *   // ...
+ * }
+ * ```
+ * You can return an object with methods `update` and `destroy` from the function.
+ * See interface `ActionReturn` for more details.
+ * 
+ * Docs: https://svelte.dev/docs#template-syntax-element-directives-use-action
+ */
+export interface Action<Element = HTMLElement, Parameter = any> {
+	<Node extends Element>(node: Node, parameter?: Parameter): void | ActionReturn<Parameter>;
+}