Export Type declarations for turbo: events

Various `turbo:`-prefixed events are dispatched as [CustomEvent][]
instances with data encoded into the [detail][] property.

In TypeScript, that property is encoded as `any`, but the `CustomEvent`
type is generic (i.e. `CustomEvent<T>`) where the generic Type argument
describes the structure of the `detail` key.

This commit introduces types that extend from `CustomEvent` for each
event, and exports them from `/core/index.ts`, which is exported from
`/index.ts` in-turn.

In practice, there are no changes to the implementation. However,
TypeScript consumers of the package can import the types. At the same
time, the internal implementation can depend on the types to ensure
consistency throughout.

[CustomEvent]: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent
[detail]: https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/detail

src/core/drive/form_submission.ts

@@ -29,6 +29,11 @@ enum FormEnctype {
   plain = "text/plain",
 }
 
+export type TurboSubmitStartEvent = CustomEvent<{ formSubmission: FormSubmission }>
+export type TurboSubmitEndEvent = CustomEvent<
+  { formSubmission: FormSubmission } & { [K in keyof FormSubmissionResult]?: FormSubmissionResult[K] }
+>
+
 function formEnctypeFromString(encoding: string): FormEnctype {
   switch (encoding.toLowerCase()) {
     case FormEnctype.multipart:
@@ -160,7 +165,7 @@ export class FormSubmission {
   requestStarted(_request: FetchRequest) {
     this.state = FormSubmissionState.waiting
     this.submitter?.setAttribute("disabled", "")
-    dispatch("turbo:submit-start", {
+    dispatch<TurboSubmitStartEvent>("turbo:submit-start", {
       target: this.formElement,
       detail: { formSubmission: this },
     })
@@ -197,7 +202,7 @@ export class FormSubmission {
   requestFinished(_request: FetchRequest) {
     this.state = FormSubmissionState.stopped
     this.submitter?.removeAttribute("disabled")
-    dispatch("turbo:submit-end", {
+    dispatch<TurboSubmitEndEvent>("turbo:submit-end", {
       target: this.formElement,
       detail: { formSubmission: this, ...this.result },
     })

src/core/frames/link_interceptor.ts

@@ -1,3 +1,5 @@
+import { TurboClickEvent, TurboBeforeVisitEvent } from "../session"
+
 export interface LinkInterceptorDelegate {
   shouldInterceptLinkClick(element: Element, url: string): boolean
   linkClickIntercepted(element: Element, url: string): void
@@ -33,7 +35,7 @@ export class LinkInterceptor {
     }
   }
 
-  linkClicked = <EventListener>((event: CustomEvent) => {
+  linkClicked = <EventListener>((event: TurboClickEvent) => {
     if (this.clickEvent && this.respondsToEventTarget(event.target) && event.target instanceof Element) {
       if (this.delegate.shouldInterceptLinkClick(event.target, event.detail.url)) {
         this.clickEvent.preventDefault()
@@ -44,9 +46,9 @@ export class LinkInterceptor {
     delete this.clickEvent
   })
 
-  willVisit = () => {
+  willVisit = <EventListener>((_event: TurboBeforeVisitEvent) => {
     delete this.clickEvent
-  }
+  })
 
   respondsToEventTarget(target: EventTarget | null) {
     const element = target instanceof Element ? target : target instanceof Node ? target.parentElement : null

src/core/index.ts

@@ -12,6 +12,21 @@ import { FormSubmission } from "./drive/form_submission"
 const session = new Session()
 const { navigator } = session
 export { navigator, session, PageRenderer, PageSnapshot, FrameRenderer }
+export {
+  TurboBeforeCacheEvent,
+  TurboBeforeRenderEvent,
+  TurboBeforeVisitEvent,
+  TurboClickEvent,
+  TurboFrameLoadEvent,
+  TurboFrameRenderEvent,
+  TurboLoadEvent,
+  TurboRenderEvent,
+  TurboVisitEvent,
+} from "./session"
+
+export { TurboSubmitStartEvent, TurboSubmitEndEvent } from "./drive/form_submission"
+export { TurboBeforeFetchRequestEvent, TurboBeforeFetchResponseEvent } from "../http/fetch_request"
+export { TurboBeforeStreamRenderEvent } from "../elements/stream_element"
 
 /**
  * Starts the main session.

src/core/session.ts

@@ -21,6 +21,15 @@ import { FetchResponse } from "../http/fetch_response"
 import { Preloader, PreloaderDelegate } from "./drive/preloader"
 
 export type TimingData = unknown
+export type TurboBeforeCacheEvent = CustomEvent
+export type TurboBeforeRenderEvent = CustomEvent<{ newBody: HTMLBodyElement; resume: (value: any) => void }>
+export type TurboBeforeVisitEvent = CustomEvent<{ url: string }>
+export type TurboClickEvent = CustomEvent<{ url: string }>
+export type TurboFrameLoadEvent = CustomEvent
+export type TurboFrameRenderEvent = CustomEvent<{ fetchResponse: FetchResponse }>
+export type TurboLoadEvent = CustomEvent<{ url: string; timing: TimingData }>
+export type TurboRenderEvent = CustomEvent
+export type TurboVisitEvent = CustomEvent<{ url: string; action: Action }>
 
 export class Session
   implements
@@ -307,43 +316,43 @@ export class Session
   }
 
   notifyApplicationAfterClickingLinkToLocation(link: Element, location: URL) {
-    return dispatch("turbo:click", {
+    return dispatch<TurboClickEvent>("turbo:click", {
       target: link,
       detail: { url: location.href },
       cancelable: true,
     })
   }
 
   notifyApplicationBeforeVisitingLocation(location: URL) {
-    return dispatch("turbo:before-visit", {
+    return dispatch<TurboBeforeVisitEvent>("turbo:before-visit", {
       detail: { url: location.href },
       cancelable: true,
     })
   }
 
   notifyApplicationAfterVisitingLocation(location: URL, action: Action) {
     markAsBusy(document.documentElement)
-    return dispatch("turbo:visit", { detail: { url: location.href, action } })
+    return dispatch<TurboVisitEvent>("turbo:visit", { detail: { url: location.href, action } })
   }
 
   notifyApplicationBeforeCachingSnapshot() {
-    return dispatch("turbo:before-cache")
+    return dispatch<TurboBeforeCacheEvent>("turbo:before-cache")
   }
 
   notifyApplicationBeforeRender(newBody: HTMLBodyElement, resume: (value: any) => void) {
-    return dispatch("turbo:before-render", {
+    return dispatch<TurboBeforeRenderEvent>("turbo:before-render", {
       detail: { newBody, resume },
       cancelable: true,
     })
   }
 
   notifyApplicationAfterRender() {
-    return dispatch("turbo:render")
+    return dispatch<TurboRenderEvent>("turbo:render")
   }
 
   notifyApplicationAfterPageLoad(timing: TimingData = {}) {
     clearBusyState(document.documentElement)
-    return dispatch("turbo:load", {
+    return dispatch<TurboLoadEvent>("turbo:load", {
       detail: { url: this.location.href, timing },
     })
   }
@@ -358,11 +367,11 @@ export class Session
   }
 
   notifyApplicationAfterFrameLoad(frame: FrameElement) {
-    return dispatch("turbo:frame-load", { target: frame })
+    return dispatch<TurboFrameLoadEvent>("turbo:frame-load", { target: frame })
   }
 
   notifyApplicationAfterFrameRender(fetchResponse: FetchResponse, frame: FrameElement) {
-    return dispatch("turbo:frame-render", {
+    return dispatch<TurboFrameRenderEvent>("turbo:frame-render", {
       detail: { fetchResponse },
       target: frame,
       cancelable: true,

src/elements/stream_element.ts

@@ -1,6 +1,8 @@
 import { StreamActions } from "../core/streams/stream_actions"
 import { nextAnimationFrame } from "../util"
 
+export type TurboBeforeStreamRenderEvent = CustomEvent
+
 // <turbo-stream action=replace target=id><template>...
 
 /**
@@ -143,7 +145,7 @@ export class StreamElement extends HTMLElement {
     return (this.outerHTML.match(/<[^>]+>/) ?? [])[0] ?? "<turbo-stream>"
   }
 
-  private get beforeRenderEvent() {
+  private get beforeRenderEvent(): TurboBeforeStreamRenderEvent {
     return new CustomEvent("turbo:before-stream-render", {
       bubbles: true,
       cancelable: true,

src/http/fetch_request.ts

@@ -2,6 +2,15 @@ import { FetchResponse } from "./fetch_response"
 import { FrameElement } from "../elements/frame_element"
 import { dispatch } from "../util"
 
+export type TurboBeforeFetchRequestEvent = CustomEvent<{
+  fetchOptions: RequestInit
+  url: URL
+  resume: (value: any) => void
+}>
+export type TurboBeforeFetchResponseEvent = CustomEvent<{
+  fetchResponse: FetchResponse
+}>
+
 export interface FetchRequestDelegate {
   referrer?: URL
 
@@ -108,7 +117,7 @@ export class FetchRequest {
 
   async receive(response: Response): Promise<FetchResponse> {
     const fetchResponse = new FetchResponse(response)
-    const event = dispatch("turbo:before-fetch-response", {
+    const event = dispatch<TurboBeforeFetchResponseEvent>("turbo:before-fetch-response", {
       cancelable: true,
       detail: { fetchResponse },
       target: this.target as EventTarget,
@@ -151,7 +160,7 @@ export class FetchRequest {
 
   private async allowRequestToBeIntercepted(fetchOptions: RequestInit) {
     const requestInterception = new Promise((resolve) => (this.resolveRequestPromise = resolve))
-    const event = dispatch("turbo:before-fetch-request", {
+    const event = dispatch<TurboBeforeFetchRequestEvent>("turbo:before-fetch-request", {
       cancelable: true,
       detail: {
         fetchOptions,

src/observers/cache_observer.ts

@@ -1,3 +1,5 @@
+import { TurboBeforeCacheEvent } from "../core/session"
+
 export class CacheObserver {
   started = false
 
@@ -15,11 +17,11 @@ export class CacheObserver {
     }
   }
 
-  removeStaleElements() {
+  removeStaleElements = <EventListener>((_event: TurboBeforeCacheEvent) => {
     const staleElements = [...document.querySelectorAll('[data-turbo-cache="false"]')]
 
     for (const element of staleElements) {
       element.remove()
     }
-  }
+  })
 }

src/observers/stream_observer.ts

@@ -1,3 +1,4 @@
+import { TurboBeforeFetchResponseEvent } from "../http/fetch_request"
 import { FetchResponse } from "../http/fetch_response"
 import { StreamMessage } from "../core/streams/stream_message"
 import { StreamSource } from "../core/types"
@@ -47,7 +48,7 @@ export class StreamObserver {
     return this.sources.has(source)
   }
 
-  inspectFetchResponse = <EventListener>((event: CustomEvent) => {
+  inspectFetchResponse = <EventListener>((event: TurboBeforeFetchResponseEvent) => {
     const response = fetchResponseFromEvent(event)
     if (response && fetchResponseIsStream(response)) {
       event.preventDefault()
@@ -73,7 +74,7 @@ export class StreamObserver {
   }
 }
 
-function fetchResponseFromEvent(event: CustomEvent) {
+function fetchResponseFromEvent(event: TurboBeforeFetchResponseEvent) {
   const fetchResponse = event.detail?.fetchResponse
   if (fetchResponse instanceof FetchResponse) {
     return fetchResponse

src/util.ts

@@ -1,11 +1,14 @@
-export type DispatchOptions = {
+export type DispatchOptions<T extends CustomEvent> = {
   target: EventTarget
   cancelable: boolean
-  detail: any
+  detail: T["detail"]
 }
 
-export function dispatch(eventName: string, { target, cancelable, detail }: Partial<DispatchOptions> = {}) {
-  const event = new CustomEvent(eventName, {
+export function dispatch<T extends CustomEvent>(
+  eventName: string,
+  { target, cancelable, detail }: Partial<DispatchOptions<T>> = {}
+) {
+  const event = new CustomEvent<T["detail"]>(eventName, {
     cancelable,
     bubbles: true,
     detail,