Fix race conditions, memory leaks, and performance issues in lazy-define

src/lazy-define.ts

@@ -1,6 +1,7 @@
 type Strategy = (tagName: string) => Promise<void>
 
-const dynamicElements = new Map<string, Set<() => void>>()
+const pending = new Map<string, Set<() => void>>()
+const triggered = new Set<string>()
 
 const ready = new Promise<void>(resolve => {
   if (document.readyState !== 'loading') {
@@ -23,31 +24,67 @@ const firstInteraction = new Promise<void>(resolve => {
   document.addEventListener('pointerdown', handler, listenerOptions)
 })
 
-const visible = (tagName: string): Promise<void> =>
-  new Promise<void>(resolve => {
-    const observer = new IntersectionObserver(
-      entries => {
-        for (const entry of entries) {
-          if (entry.isIntersecting) {
-            resolve()
-            observer.disconnect()
-            return
+const visible = async (tagName: string): Promise<void> => {
+  const observeIntersection = (elements: Element[]) => {
+    return new Promise<void>(resolve => {
+      const observer = new IntersectionObserver(
+        entries => {
+          for (const entry of entries) {
+            if (entry.isIntersecting) {
+              resolve()
+              observer.disconnect()
+              return
+            }
           }
+        },
+        {
+          // Currently the threshold is set to 256px from the bottom of the viewport
+          // with a threshold of 0.1. This means the element will not load until about
+          // 2 keyboard-down-arrow presses away from being visible in the viewport,
+          // giving us some time to fetch it before the contents are made visible
+          rootMargin: '0px 0px 256px 0px',
+          threshold: 0.01
         }
-      },
-      {
-        // Currently the threshold is set to 256px from the bottom of the viewport
-        // with a threshold of 0.1. This means the element will not load until about
-        // 2 keyboard-down-arrow presses away from being visible in the viewport,
-        // giving us some time to fetch it before the contents are made visible
-        rootMargin: '0px 0px 256px 0px',
-        threshold: 0.01
+      )
+      for (const element of elements) {
+        observer.observe(element)
       }
-    )
-    for (const el of document.querySelectorAll(tagName)) {
-      observer.observe(el)
-    }
-  })
+    })
+  }
+
+  const waitForElement = () => {
+    return new Promise<Element[]>(resolve => {
+      const observer = new MutationObserver(mutations => {
+        for (const mutation of mutations) {
+          const addedNodes = Array.from(mutation.addedNodes)
+          for (const node of addedNodes) {
+            if (!(node instanceof Element)) continue
+
+            const isMatch = node.matches(tagName)
+            const descendant = node.querySelector(tagName)
+
+            if (isMatch || descendant) {
+              observer.disconnect()
+              resolve(Array.from(document.querySelectorAll(tagName)))
+              return
+            }
+          }
+        }
+      })
+
+      observer.observe(document.documentElement, {childList: true, subtree: true})
+    })
+  }
+
+  const existingElements = Array.from(document.querySelectorAll(tagName))
+
+  if (existingElements.length > 0) {
+    return observeIntersection(existingElements)
+  }
+
+  const foundElements = await waitForElement()
+  return observeIntersection(foundElements)
+}
 
 const strategies: Record<string, Strategy> = {
   ready: () => ready,
@@ -57,54 +94,121 @@ const strategies: Record<string, Strategy> = {
 
 type ElementLike = Element | Document | ShadowRoot
 
+const observedTargets = new WeakSet<ElementLike>()
 const timers = new WeakMap<ElementLike, number>()
+
+function cleanupObserver() {
+  if (pending.size === 0 && elementLoader) {
+    elementLoader.disconnect()
+    elementLoader = undefined
+  }
+}
+
 function scan(element: ElementLike) {
-  cancelAnimationFrame(timers.get(element) || 0)
-  timers.set(
-    element,
-    requestAnimationFrame(() => {
-      for (const tagName of dynamicElements.keys()) {
-        const child: Element | null =
-          element instanceof Element && element.matches(tagName) ? element : element.querySelector(tagName)
-        if (customElements.get(tagName) || child) {
-          const strategyName = (child?.getAttribute('data-load-on') || 'ready') as keyof typeof strategies
-          const strategy = strategyName in strategies ? strategies[strategyName] : strategies.ready
-          // eslint-disable-next-line github/no-then
-          for (const cb of dynamicElements.get(tagName) || []) strategy(tagName).then(cb)
-          dynamicElements.delete(tagName)
-          timers.delete(element)
+  const currentTimer = timers.get(element)
+  if (currentTimer) cancelAnimationFrame(currentTimer)
+
+  const newTimer = requestAnimationFrame(() => {
+    // FIX 7: Early return optimization
+    if (pending.size === 0) return
+
+    // FIX 7: Create snapshot to prevent modification-during-iteration issues
+    // (concurrent scans may delete tags from pending)
+    const tagList = Array.from(pending.keys())
+
+    for (const tagName of tagList) {
+      const child: Element | null =
+        element instanceof Element && element.matches(tagName) ? element : element.querySelector(tagName)
+      if (customElements.get(tagName) || child) {
+        // Skip if already processed and not re-registered
+        if (triggered.has(tagName) && !pending.has(tagName)) continue
+
+        triggered.add(tagName)
+
+        const callbackSet = pending.get(tagName)
+        pending.delete(tagName)
+
+        const strategyName = (child?.getAttribute('data-load-on') || 'ready') as keyof typeof strategies
+        const strategy = strategyName in strategies ? strategies[strategyName] : strategies.ready
+
+        // FIX 5: Wrap callback execution in try-catch and handle rejections
+        const callbackList = Array.from(callbackSet || [])
+        for (const callback of callbackList) {
+          strategy(tagName)
+            // eslint-disable-next-line github/no-then
+            .then(() => {
+              try {
+                callback()
+              } catch (err) {
+                reportError(err)
+              }
+            })
+            // eslint-disable-next-line github/no-then
+            .catch(reportError)
         }
+
+        timers.delete(element)
       }
-    })
-  )
+    }
+
+    // FIX 4: Disconnect observer when all pending tags are processed
+    cleanupObserver()
+  })
+
+  timers.set(element, newTimer)
 }
 
-let elementLoader: MutationObserver
+let elementLoader: MutationObserver | undefined
 
 export function lazyDefine(object: Record<string, () => void>): void
 export function lazyDefine(tagName: string, callback: () => void): void
 export function lazyDefine(tagNameOrObj: string | Record<string, () => void>, singleCallback?: () => void) {
   if (typeof tagNameOrObj === 'string' && singleCallback) {
     tagNameOrObj = {[tagNameOrObj]: singleCallback}
   }
+
   for (const [tagName, callback] of Object.entries(tagNameOrObj)) {
-    if (!dynamicElements.has(tagName)) dynamicElements.set(tagName, new Set<() => void>())
-    dynamicElements.get(tagName)!.add(callback)
+    // FIX 6: Late registration - execute immediately if already triggered
+    // Check both triggered state and element existence to avoid executing for removed elements
+    if (triggered.has(tagName) && document.querySelector(tagName)) {
+      // eslint-disable-next-line github/no-then
+      Promise.resolve().then(() => {
+        try {
+          callback()
+        } catch (err) {
+          reportError(err)
+        }
+      })
+    } else {
+      if (!pending.has(tagName)) {
+        pending.set(tagName, new Set<() => void>())
+      }
+      pending.get(tagName)!.add(callback)
+    }
   }
   observe(document)
 }
 
 export function observe(target: ElementLike): void {
-  elementLoader ||= new MutationObserver(mutations => {
-    if (!dynamicElements.size) return
-    for (const mutation of mutations) {
-      for (const node of mutation.addedNodes) {
-        if (node instanceof Element) scan(node)
+  if (!elementLoader) {
+    elementLoader = new MutationObserver(mutations => {
+      if (!pending.size) return
+      for (const mutation of mutations) {
+        const nodes = mutation.addedNodes
+        for (const node of nodes) {
+          if (node instanceof Element) {
+            scan(node)
+          }
+        }
       }
-    }
-  })
+    })
+  }
 
   scan(target)
 
-  elementLoader.observe(target, {subtree: true, childList: true})
+  // FIX 3: Check observedTargets to avoid redundant observe() calls
+  if (!observedTargets.has(target)) {
+    observedTargets.add(target)
+    elementLoader.observe(target, {subtree: true, childList: true})
+  }
 }