Respect forms with enctype set for view transitions

packages/astro/components/ViewTransitions.astro

@@ -117,7 +117,18 @@ const { fallback = 'animate' } = Astro.props;
 				url.search = params.toString();
 				action = url.toString();
 			} else {
-				options.formData = formData;
+				// Form elements without enctype explicitly set default to application/x-www-form-urlencoded.
+				// In order to maintain compatibility with Astro 4.x, we need to check the value of enctype
+				// on the attributes property rather than accessing .enctype directly. Astro 5.x may
+				// introduce defaulting to application/x-www-form-urlencoded as a breaking change, and then
+				// we can access .enctype directly.
+				//
+				// Note: getNamedItem can return null in real life, even if TypeScript doesn't think so.
+				const enctype = form.attributes.getNamedItem('enctype')?.value;
+				options.body =
+					enctype === 'application/x-www-form-urlencoded'
+						? new URLSearchParams(formData as any)
+						: formData;
 			}
 
 			ev.preventDefault();

packages/astro/src/transitions/events.ts

@@ -66,7 +66,7 @@ export const isTransitionBeforePreparationEvent = (
 	value: any
 ): value is TransitionBeforePreparationEvent => value.type === TRANSITION_BEFORE_PREPARATION;
 export class TransitionBeforePreparationEvent extends BeforeEvent {
-	formData: FormData | undefined;
+	body: FormData | URLSearchParams | undefined;
 	loader: () => Promise<void>;
 	constructor(
 		from: URL,
@@ -76,7 +76,7 @@ export class TransitionBeforePreparationEvent extends BeforeEvent {
 		sourceElement: Element | undefined,
 		info: any,
 		newDocument: Document,
-		formData: FormData | undefined,
+		body: FormData | URLSearchParams | undefined,
 		loader: (event: TransitionBeforePreparationEvent) => Promise<void>
 	) {
 		super(
@@ -90,7 +90,7 @@ export class TransitionBeforePreparationEvent extends BeforeEvent {
 			info,
 			newDocument
 		);
-		this.formData = formData;
+		this.body = body;
 		this.loader = loader.bind(this, this);
 		Object.defineProperties(this, {
 			formData: { enumerable: true },
@@ -145,7 +145,7 @@ export async function doPreparation(
 	navigationType: NavigationTypeString,
 	sourceElement: Element | undefined,
 	info: any,
-	formData: FormData | undefined,
+	body: FormData | URLSearchParams | undefined,
 	defaultLoader: (event: TransitionBeforePreparationEvent) => Promise<void>
 ) {
 	const event = new TransitionBeforePreparationEvent(
@@ -156,7 +156,7 @@ export async function doPreparation(
 		sourceElement,
 		info,
 		window.document,
-		formData,
+		body,
 		defaultLoader
 	);
 	if (document.dispatchEvent(event)) {

packages/astro/src/transitions/router.ts

@@ -449,7 +449,7 @@ async function transition(
 		navigationType,
 		options.sourceElement,
 		options.info,
-		options.formData,
+		options.body,
 		defaultLoader
 	);
 	if (prepEvent.defaultPrevented) {
@@ -460,9 +460,9 @@ async function transition(
 	async function defaultLoader(preparationEvent: TransitionBeforePreparationEvent) {
 		const href = preparationEvent.to.href;
 		const init: RequestInit = {};
-		if (preparationEvent.formData) {
+		if (preparationEvent.body) {
 			init.method = 'POST';
-			init.body = preparationEvent.formData;
+			init.body = preparationEvent.body;
 		}
 		const response = await fetchHTML(href, init);
 		// If there is a problem fetching the new page, just do an MPA navigation to it.
@@ -488,7 +488,7 @@ async function transition(
 		// Unless this was a form submission, in which case we do not want to trigger another mutation.
 		if (
 			!preparationEvent.newDocument.querySelector('[name="astro-view-transitions-enabled"]') &&
-			!preparationEvent.formData
+			!preparationEvent.body
 		) {
 			preparationEvent.preventDefault();
 			return;

packages/astro/src/transitions/types.ts

@@ -5,6 +5,6 @@ export type Options = {
 	history?: 'auto' | 'push' | 'replace';
 	info?: any;
 	state?: any;
-	formData?: FormData;
+	body?: FormData | URLSearchParams;
 	sourceElement?: Element; // more than HTMLElement, e.g. SVGAElement
 };