feat: add `beforeMount` callback to `MountOptions`

.changeset/cyan-kings-wait.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: add `beforeMount` callback to `MountOptions`

packages/svelte/src/index.d.ts

@@ -333,6 +333,10 @@ export type MountOptions<Props extends Record<string, any> = Record<string, any>
 	 * Can be accessed via `getContext()` at the component level.
 	 */
 	context?: Map<any, any>;
+	/**
+	 * A function invoked before the component is mounted but after the component context has been initialized.
+	 */
+	beforeMount?: () => void;
 	/**
 	 * Whether or not to play transitions on initial render.
 	 * @default true

packages/svelte/src/internal/client/render.js

@@ -158,7 +158,10 @@ const document_listeners = new Map();
  * @param {MountOptions} options
  * @returns {Exports}
  */
-function _mount(Component, { target, anchor, props = {}, events, context, intro = true }) {
+function _mount(
+	Component,
+	{ target, anchor, props = {}, beforeMount, events, context, intro = true }
+) {
 	init_operations();
 
 	/** @type {Set<string>} */
@@ -214,6 +217,8 @@ function _mount(Component, { target, anchor, props = {}, events, context, intro
 					ctx.c = context;
 				}
 
+				beforeMount?.();
+
 				if (events) {
 					// We can't spread the object or else we'd lose the state proxy stuff, if it is one
 					/** @type {any} */ (props).$$events = events;