Added more clarification around using Astro.slots.render (#4456)

* Add an error message for when something that's not an array is passed to Astro.slots.render

* Add changeset

* Add more details

.changeset/unlucky-otters-end.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Added an error message when the second argument of Astro.slots.render is not an array

packages/astro/src/@types/astro.ts

@@ -203,7 +203,7 @@ export interface AstroGlobal extends AstroGlobalPartial {
 		 */
 		has(slotName: string): boolean;
 		/**
-		 * Asychronously renders this slot and returns HTML
+		 * Asynchronously renders this slot and returns a string
 		 *
 		 * Example usage:
 		 * ```astro
@@ -216,6 +216,21 @@ export interface AstroGlobal extends AstroGlobalPartial {
 		 * <Fragment set:html={html} />
 		 * ```
 		 *
+		 * A second parameters can be used to pass arguments to a slotted callback
+		 *
+		 * Example usage:
+		 * ```astro
+		 * ---
+		 * html = await Astro.slots.render('default', ["Hello", "World"])
+		 * ---
+		 * ```
+		 * Each item in the array will be passed as an argument that you can use like so:
+		 * ```astro
+		 * <Component>
+		 *		{(hello, world) => <div>{hello}, {world}!</div>}
+		 * </Component>
+		 * ```
+		 *
 		 * [Astro reference](https://docs.astro.build/en/reference/api-reference/#astroslots)
 		 */
 		render(slotName: string, args?: any[]): Promise<string>;

packages/astro/src/core/render/result.ts

@@ -55,10 +55,13 @@ class Slots {
 	#cache = new Map<string, string>();
 	#result: SSRResult;
 	#slots: Record<string, any> | null;
+	#loggingOpts: LogOptions;
 
-	constructor(result: SSRResult, slots: Record<string, any> | null) {
+	constructor(result: SSRResult, slots: Record<string, any> | null, logging: LogOptions) {
 		this.#result = result;
 		this.#slots = slots;
+		this.#loggingOpts = logging;
+
 		if (slots) {
 			for (const key of Object.keys(slots)) {
 				if ((this as any)[key] !== undefined) {
@@ -92,11 +95,20 @@ class Slots {
 		if (!cacheable) {
 			const component = await this.#slots[name]();
 			const expression = getFunctionExpression(component);
-			if (expression) {
-				const slot = expression(...args);
-				return await renderSlot(this.#result, slot).then((res) =>
-					res != null ? String(res) : res
+
+			if (!Array.isArray(args)) {
+				warn(
+					this.#loggingOpts,
+					'Astro.slots.render',
+					`Expected second parameter to be an array, received a ${typeof args}. If you're trying to pass an array as a single argument and getting unexpected results, make sure you're passing your array as a item of an array. Ex: Astro.slots.render('default', [["Hello", "World"]])`
 				);
+			} else {
+				if (expression) {
+					const slot = expression(...args);
+					return await renderSlot(this.#result, slot).then((res) =>
+						res != null ? String(res) : res
+					);
+				}
 			}
 		}
 		const content = await renderSlot(this.#result, this.#slots[name]).then((res) =>
@@ -146,7 +158,7 @@ export function createResult(args: CreateResultArgs): SSRResult {
 			props: Record<string, any>,
 			slots: Record<string, any> | null
 		) {
-			const astroSlots = new Slots(result, slots);
+			const astroSlots = new Slots(result, slots, args.logging);
 
 			const Astro = {
 				__proto__: astroGlobal,