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>} */
@@ -208,12 +211,16 @@ function _mount(Component, { target, anchor, props = {}, events, context, intro
 				pending: () => {}
 			},
 			(anchor_node) => {
-				if (context) {
+				if (context || beforeMount) {
 					push({});
-					var ctx = /** @type {ComponentContext} */ (component_context);
-					ctx.c = context;
+					if (context) {
+						var ctx = /** @type {ComponentContext} */ (component_context);
+						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;
@@ -241,7 +248,7 @@ function _mount(Component, { target, anchor, props = {}, events, context, intro
 					}
 				}
 
-				if (context) {
+				if (context || beforeMount) {
 					pop();
 				}
 			}

packages/svelte/types/index.d.ts

@@ -332,6 +332,10 @@ declare module 'svelte' {
 		 * 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