Extract getAttribute utility function

For cases where we need to find an attribute value from a collection of
elements, use `getAttribute` instead of a long chain of `||` and `?`
operators.

src/core/drive/navigator.ts

@@ -3,6 +3,7 @@ import { FetchMethod } from "../../http/fetch_request"
 import { FetchResponse } from "../../http/fetch_response"
 import { FormSubmission } from "./form_submission"
 import { expandURL, getAnchor, getRequestURL, Locatable, locationIsVisitable } from "../url"
+import { getAttribute } from "../../util"
 import { Visit, VisitDelegate, VisitOptions } from "./visit"
 import { PageSnapshot } from "./page_snapshot"
 
@@ -158,7 +159,7 @@ export class Navigator {
 
   getActionForFormSubmission(formSubmission: FormSubmission): Action {
     const { formElement, submitter } = formSubmission
-    const action = submitter?.getAttribute("data-turbo-action") || formElement.getAttribute("data-turbo-action")
+    const action = getAttribute("data-turbo-action", submitter, formElement)
     return isAction(action) ? action : "advance"
   }
 }

src/core/frames/frame_controller.ts

@@ -2,7 +2,7 @@ import { FrameElement, FrameElementDelegate, FrameLoadingStyle } from "../../ele
 import { FetchMethod, FetchRequest, FetchRequestDelegate, FetchRequestHeaders } from "../../http/fetch_request"
 import { FetchResponse } from "../../http/fetch_response"
 import { AppearanceObserver, AppearanceObserverDelegate } from "../../observers/appearance_observer"
-import { parseHTMLDocument } from "../../util"
+import { getAttribute, parseHTMLDocument } from "../../util"
 import { FormSubmission, FormSubmissionDelegate } from "../drive/form_submission"
 import { Snapshot } from "../snapshot"
 import { ViewDelegate } from "../view"
@@ -259,7 +259,7 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
   }
 
   private proposeVisitIfNavigatedWithAction(frame: FrameElement, element: Element, submitter?: HTMLElement) {
-    const action = submitter?.getAttribute("data-turbo-action") || element.getAttribute("data-turbo-action") || frame.getAttribute("data-turbo-action")
+    const action = getAttribute("data-turbo-action", submitter, element, frame)
 
     if (isAction(action)) {
       const proposeVisit = async (event: Event) => {
@@ -274,7 +274,7 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
   }
 
   private findFrameElement(element: Element, submitter?: HTMLElement) {
-    const id = submitter?.getAttribute("data-turbo-frame") || element.getAttribute("data-turbo-frame") || this.element.getAttribute("target")
+    const id = getAttribute("data-turbo-frame", submitter, element) || this.element.getAttribute("target")
     return getFrameElementById(id) ?? this.element
   }
 
@@ -307,7 +307,7 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
   }
 
   private shouldInterceptNavigation(element: Element, submitter?: HTMLElement) {
-    const id = submitter?.getAttribute("data-turbo-frame") || element.getAttribute("data-turbo-frame") || this.element.getAttribute("target")
+    const id = getAttribute("data-turbo-frame", submitter, element) || this.element.getAttribute("target")
 
     if (element instanceof HTMLFormElement && !this.formActionIsVisitable(element, submitter)) {
       return false

src/util.ts

@@ -55,3 +55,11 @@ export function uuid() {
     }
   }).join("")
 }
+
+export function getAttribute(attributeName: string, ...elements: (Element | undefined)[]): string | null {
+  for (const value of elements.map(element => element?.getAttribute(attributeName))) {
+    if (typeof value == "string") return value
+  }
+
+  return null
+}