JavaScript Performance Optimization for the Web

JavaScript execution is the primary source of main-thread blocking in modern web applications. This article covers the browser’s script loading pipeline, task scheduling primitives, code splitting strategies, and Web Workers—with emphasis on design rationale and current API status as of 2025.

Abstract

JavaScript performance optimization addresses three distinct bottlenecks:

1. Parse-time blocking: Scripts block DOM construction during fetch and execution. The async, defer, and type="module" attributes control when scripts execute relative to parsing—defer guarantees document order after parsing completes, async executes on arrival (no order guarantee), and modules are deferred by default with dependency resolution.

2. Run-time blocking: Tasks exceeding 50ms (the RAIL-derived threshold for perceived responsiveness) block input handling. scheduler.yield() (Chrome 129+) yields to the browser while maintaining task priority through a continuation queue mechanism—unlike setTimeout(0) which sends work to the back of the queue.

3. Bundle-size blocking: Large initial bundles delay Time to Interactive (TTI). Code splitting via dynamic import() defers non-critical code; tree shaking eliminates dead exports statically. Both require ES modules—CommonJS is runtime-resolved and cannot be statically analyzed.

Web Workers move computation entirely off the main thread. Transferable objects enable zero-copy data transfer by detaching ownership from the sending context.

Script Loading: Parser Interaction and Execution Order

The choice of script loading strategy determines when the browser’s HTML parser yields control to JavaScript execution.

Why Parser Blocking Matters

The HTML parser cannot construct DOM nodes while JavaScript executes—JavaScript may call document.write() or modify the DOM being built. This creates the fundamental tension: scripts need DOM access, but parsing must complete for DOM to exist.

WHATWG HTML spec: “The classic script will be fetched in parallel and evaluated when the page has finished parsing.”

This quote describes defer behavior—the spec’s solution to the tension. The parser continues unblocked while scripts download, then scripts execute in document order after parsing.

Execution Order Guarantees by Attribute

Why defer Executes After Parsing

The defer attribute was introduced in Internet Explorer 4 (1997) and standardized in HTML 4. The design rationale: hint to the browser that the script “does not create any content” via document.write(), so parsing can continue.

Prior to defer: All scripts blocked parsing during both download and execution. Users saw nothing until scripts completed—a significant perceived performance problem on slow networks.

The parser maintains a “list of scripts that will execute when the document has finished parsing.” Deferred scripts append to this list and execute sequentially, preserving document order.

async vs defer Decision Matrix

Module Scripts: Deferred by Default

ES modules (type="module") are deferred by default with additional semantics:

• Strict mode: Always enabled, this is undefined at top level
• Dependency resolution: The module graph is fetched before any execution
• Top-level await: Supported (blocks dependent modules, not the parser)

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<!-- Analytics: independent, no DOM dependency -->
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<script src="analytics.js" async></script>
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<!-- Polyfills → Framework → App: order-dependent -->
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<script src="polyfills.js" defer></script>
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<script src="framework.js" defer></script>
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<script src="app.js" defer></script>
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<!-- Modern entry: ESM with automatic deferral -->
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<script type="module" src="main.js"></script>

Trade-off: Module scripts require nomodule fallback for legacy browsers (IE11, older Safari), adding bundle complexity.

Long Task Management: The 50ms Threshold

Tasks exceeding 50ms are classified as “long tasks” and directly impact Interaction to Next Paint (INP).

Why 50ms?

The threshold derives from the RAIL performance model’s responsiveness target:

W3C Long Tasks spec: “Any of the following occurrences whose duration exceeds 50ms.”

The reasoning:

• Users perceive responses under 100ms as instantaneous
• A 50ms task budget leaves 50ms for the browser to process input and render
• This 50ms + 50ms = 100ms total maintains perceived responsiveness

W3C requestIdleCallback spec: “The maximum deadline of 50ms is derived from studies which show that a response to user input within 100ms is generally perceived as instantaneous.”

scheduler.yield(): Priority-Preserving Yielding

As of Chrome 129 (September 2024), scheduler.yield() provides yielding with priority inheritance.

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async function processLargeDataset(items) {
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  const results = []
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  const BATCH_SIZE = 50
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  for (let i = 0; i < items.length; i++) {
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    results.push(await computeExpensiveOperation(items[i]))
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    // Yield every 50 items, maintaining task priority
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    if (i % BATCH_SIZE === 0 && i > 0) {
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      await scheduler.yield()
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    }
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  }
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  return results
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}

Why Not setTimeout(0)?

setTimeout(0) has critical limitations that scheduler.yield() addresses:

WICG Scheduling spec: “Continuations have a higher effective priority than tasks with the same TaskPriority.”

The browser maintains separate continuation queues for each priority level. A user-visible continuation runs before user-visible regular tasks but after user-blocking tasks.

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// Priority inheritance demonstration
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scheduler.postTask(
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  async () => {
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    // This runs at background priority
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    await heavyComputation()
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    // After yield, still at background priority
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    // But ahead of OTHER background tasks
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    await scheduler.yield()
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    await moreComputation()
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  },
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  { priority: "background" },
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)

Adaptive Time-Slice Yielding

For variable-cost work items, yield based on elapsed time rather than iteration count:

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async function adaptiveProcessing(workQueue) {
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  const TIME_SLICE_MS = 5
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  while (workQueue.length > 0) {
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    const sliceStart = performance.now()
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    // Process until time slice exhausted
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    while (workQueue.length > 0 && performance.now() - sliceStart < TIME_SLICE_MS) {
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      processWorkItem(workQueue.shift())
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    }
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    if (workQueue.length > 0) {
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      await scheduler.yield()
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    }
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  }
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}

Browser Support and Fallback

For browsers without support, fall back to setTimeout(0) with the understanding that priority is lost:

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const yieldToMain = () => {
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  if ("scheduler" in globalThis && "yield" in scheduler) {
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    return scheduler.yield()
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  }
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  return new Promise((resolve) => setTimeout(resolve, 0))
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}

Code Splitting: Reducing Initial Bundle Size

Code splitting defers loading non-critical code until needed, reducing Time to Interactive (TTI).

Route-Based Splitting

Route-based splitting is the highest-impact strategy—each route loads only its required code:

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import { lazy, Suspense } from "react"
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import { Routes, Route } from "react-router-dom"
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const Home = lazy(() => import("./pages/Home"))
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const Dashboard = lazy(() => import("./pages/Dashboard"))
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const Settings = lazy(() => import("./pages/Settings"))
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function App() {
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  return (
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    <Suspense fallback={<LoadingSpinner />}>
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      <Routes>
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        <Route path="/" element={<Home />} />
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        <Route path="/dashboard" element={<Dashboard />} />
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        <Route path="/settings" element={<Settings />} />
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      </Routes>
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    </Suspense>
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  )
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}

The Chunk Loading Waterfall Problem

Sequential chunk loading creates waterfalls:

1. Download and parse entry.js
2. Router determines current route
3. Download route chunk
4. Parse and hydrate

Solution: Inject a preload script that runs before the main bundle:

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<script>
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  const routeChunks = { "/": "home.js", "/dashboard": "dashboard.js" }
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  const chunk = routeChunks[location.pathname]
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  if (chunk) {
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    const link = document.createElement("link")
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    link.rel = "preload"
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    link.as = "script"
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    link.href = `/chunks/${chunk}`
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    document.head.appendChild(link)
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  }
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</script>

webpackPrefetch vs webpackPreload

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// Preload: needed NOW, loads in parallel
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import(/* webpackPreload: true */ "ChartingLibrary")
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// Prefetch: needed LATER, loads during idle
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import(/* webpackPrefetch: true */ "./SettingsPage")

Trade-off: Overusing preload competes for bandwidth with critical resources. Prefetch is safer—it uses idle time but may not complete before needed.

Component-Level Splitting

Split individual heavy components (>30KB) that aren’t needed immediately:

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import { lazy, Suspense, useState, startTransition } from "react"
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const HeavyChart = lazy(() => import("./HeavyChart"))
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function Dashboard() {
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  const [showChart, setShowChart] = useState(false)
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  const loadChart = () => {
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    // Use transition to avoid blocking UI
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    startTransition(() => setShowChart(true))
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  }
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  return (
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    <div>
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      <button onClick={loadChart}>Show Analytics</button>
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      {showChart && (
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        <Suspense fallback={<ChartSkeleton />}>
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          <HeavyChart />
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        </Suspense>
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      )}
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    </div>
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  )
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}

Tree Shaking: Dead Code Elimination

Tree shaking removes unused exports from the bundle at build time.

Why ES Modules Enable Tree Shaking

ES modules are statically analyzable—imports and exports can be determined at compile time without executing code.

ECMA-262 design: Import/export declarations can only appear at module top level, and specifiers must be string literals.

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// ✅ Static - bundler knows exactly what's used
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import { add, multiply } from "./math.js"
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// ❌ Dynamic - cannot analyze at build time
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const fn = condition ? "add" : "multiply"
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import("./math.js").then((mod) => mod[fn]())

CommonJS cannot be tree-shaken because require() is a runtime function that can be called conditionally with computed paths.

When Tree Shaking Fails

1. Side effects present: Code that runs on import (polyfills, CSS, prototype modifications)
2. Dynamic property access: utils[methodName]
3. Dynamic imports: The entire module is included
4. Missing sideEffects field: Bundler assumes all files have side effects

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// package.json - declare no side effects for aggressive tree shaking
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{
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  "sideEffects": false
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}
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// Or whitelist files with side effects
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{
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  "sideEffects": ["*.css", "./src/polyfills.js"]
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}

Critical pitfall: sideEffects: false without whitelisting CSS files will remove all CSS imports.

Bundler Differences

esbuild interprets sideEffects: false narrowly—it removes entire unused modules but doesn’t tree-shake individual statements within modules.

Web Workers: Off-Main-Thread Computation

Web Workers run JavaScript in background threads, preventing long computations from blocking the main thread.

When to Use Workers

Workers have overhead: message serialization, thread creation, no DOM access. Use them for:

• CPU-intensive computation: Image processing, encryption, compression
• Large data processing: Sorting, filtering, aggregation
• Background sync: Data transformation while UI remains responsive

WHATWG Workers spec: “Workers are relatively heavy-weight, and are not intended to be used in large numbers.”

Basic Worker Pattern

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const worker = new Worker("worker.js")
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worker.postMessage({ type: "PROCESS", data: largeDataset })
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worker.onmessage = (event) => {
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  if (event.data.type === "COMPLETE") {
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    updateUI(event.data.result)
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  }
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}
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worker.onerror = (error) => {
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  console.error("Worker error:", error.message)
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}

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self.onmessage = (event) => {
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  const { type, data } = event.data
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  if (type === "PROCESS") {
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    try {
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      const result = expensiveComputation(data)
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      self.postMessage({ type: "COMPLETE", result })
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    } catch (error) {
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      self.postMessage({ type: "ERROR", message: error.message })
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    }
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  }
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}

Transferable Objects: Zero-Copy Transfer

By default, postMessage uses the structured clone algorithm—deep copying data. For large ArrayBuffers, use transfer instead:

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const buffer = new ArrayBuffer(1024 * 1024 * 100) // 100MB
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console.log(buffer.byteLength) // 104857600
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// Transfer ownership - zero copy
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worker.postMessage({ buffer }, [buffer])
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console.log(buffer.byteLength) // 0 (detached)

WHATWG spec: “Transferring is an irreversible and non-idempotent operation. Once transferred, an object cannot be transferred or used again.”

Transferable types: ArrayBuffer, MessagePort, ImageBitmap, OffscreenCanvas, ReadableStream, WritableStream, TransformStream, VideoFrame, AudioData.

Common mistake: TypedArrays (e.g., Uint8Array) are not transferable—only their underlying ArrayBuffer is.

Error Handling Edge Cases

Synchronous exceptions propagate to the parent via worker.onerror:

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worker.onerror = (event) => {
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  console.error(`Worker error: ${event.message} at ${event.filename}:${event.lineno}`)
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  event.preventDefault() // Prevents default error logging
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}

Unhandled promise rejections do NOT propagate to the parent—they only log to the worker’s console. Implement explicit error messaging:

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self.addEventListener("unhandledrejection", (event) => {
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  self.postMessage({
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    type: "ERROR",
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    message: event.reason?.message || "Unhandled rejection",
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  })
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})

Worker Pool for Parallel Processing

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class WorkerPool {
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  constructor(workerScript, poolSize = navigator.hardwareConcurrency) {
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    this.workers = []
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    this.queue = []
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    this.available = []
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    for (let i = 0; i < poolSize; i++) {
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      const worker = new Worker(workerScript)
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      worker.onmessage = (e) => this.handleMessage(worker, e)
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      worker.onerror = (e) => this.handleError(worker, e)
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      this.workers.push(worker)
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      this.available.push(worker)
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    }
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  }
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  execute(task) {
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    return new Promise((resolve, reject) => {
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      const wrapper = { task, resolve, reject }
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      if (this.available.length > 0) {
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        this.dispatch(this.available.pop(), wrapper)
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      } else {
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        this.queue.push(wrapper)
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      }
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    })
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  }
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  dispatch(worker, wrapper) {
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    worker.currentTask = wrapper
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    worker.postMessage(wrapper.task)
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  }
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  handleMessage(worker, event) {
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    const { resolve, reject } = worker.currentTask
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    if (event.data.error) {
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      reject(new Error(event.data.error))
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    } else {
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      resolve(event.data.result)
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    }
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    // Return to pool or process queue
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    if (this.queue.length > 0) {
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      this.dispatch(worker, this.queue.shift())
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    } else {
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      this.available.push(worker)
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    }
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  }
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  handleError(worker, event) {
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    worker.currentTask?.reject(new Error(event.message))
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    // Worker may be unusable - consider recreating
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  }
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  terminate() {
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    this.workers.forEach((w) => w.terminate())
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  }
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}

SharedArrayBuffer and Cross-Origin Isolation

SharedArrayBuffer enables shared memory between workers but requires cross-origin isolation due to Spectre/Meltdown mitigations:

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Cross-Origin-Opener-Policy: same-origin
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Cross-Origin-Embedder-Policy: require-corp

Why the restriction: SharedArrayBuffer enables construction of nanosecond-resolution timers that can be used for cache timing attacks to extract cross-origin data.

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// Verify cross-origin isolation
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if (crossOriginIsolated) {
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  const shared = new SharedArrayBuffer(1024)
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  // Can use Atomics for synchronization
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} else {
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  console.warn("SharedArrayBuffer unavailable - not cross-origin isolated")
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}

Safari limitation: Safari does not support COEP: credentialless, requiring the stricter require-corp mode where all cross-origin resources must explicitly opt-in with CORS or CORP headers.

React Optimization Patterns

React applications have framework-specific optimization opportunities.

React.memo: Preventing Unnecessary Re-renders

React.memo creates a higher-order component that skips re-rendering when props are shallowly equal:

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const ExpensiveList = React.memo(function ExpensiveList({ items, onSelect }) {
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  return (
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    <ul>
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      {items.map((item) => (
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        <li key={item.id} onClick={() => onSelect(item.id)}>
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          {item.name}
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        </li>
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      ))}
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    </ul>
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  )
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})
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// Custom comparison for complex props
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const MemoizedChart = React.memo(
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  function Chart({ data, config }) {
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    return <ChartImpl data={data} config={config} />
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  },
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  (prev, next) => {
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    return prev.data.length === next.data.length && prev.config.type === next.config.type
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  },
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)

When to use: Components that receive the same props frequently, expensive render logic, components deep in the tree that re-render due to parent updates.

When NOT to use: Components that always receive new props (breaks memoization), simple components (memo overhead exceeds render cost).

useMemo and useCallback: Stabilizing References

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function DataGrid({ data, filters, onRowClick }) {
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  const [sortConfig, setSortConfig] = useState({ key: "id", direction: "asc" })
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  // Memoize expensive computation
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  const filteredData = useMemo(() => {
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    return data.filter((item) => matchesFilters(item, filters)).sort((a, b) => compareBy(a, b, sortConfig))
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  }, [data, filters, sortConfig])
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  // Stabilize callback reference for memoized children
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  const handleRowClick = useCallback(
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    (rowId) => {
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      onRowClick(rowId)
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    },
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    [onRowClick],
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  )
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  return <MemoizedTable data={filteredData} onRowClick={handleRowClick} />
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}

React Server Components

As of React 18, Server Components run on the server with zero client bundle impact:

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// No 'use client' - runs on server only
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import { db } from "./database" // Server-only module
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import ClientChart from "./ClientChart"
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export default async function Dashboard({ userId }) {
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  // Direct database access - no API needed
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  const metrics = await db.query("SELECT * FROM metrics WHERE user_id = ?", [userId])
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  return (
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    <div>
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      <h1>Dashboard</h1>
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      {/* ClientChart creates a split point */}
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      <ClientChart data={metrics} />
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    </div>
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  )
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}

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"use client"
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import { useState } from "react"
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export default function ClientChart({ data }) {
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  const [zoom, setZoom] = useState(1)
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  return <canvas data-zoom={zoom} />
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}

Key insight: The 'use client' directive creates an automatic code split point. Server-side dependencies (database clients, large processing libraries) never ship to the client.

Virtualization for Large Lists

For lists with thousands of items, render only visible items:

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import { useVirtualizer } from "@tanstack/react-virtual"
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function VirtualList({ items }) {
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  const parentRef = useRef(null)
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  const virtualizer = useVirtualizer({
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    count: items.length,
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    getScrollElement: () => parentRef.current,
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    estimateSize: () => 50, // Estimated row height
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    overscan: 5, // Render 5 extra items above/below viewport
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  })
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  return (
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    <div ref={parentRef} style={{ height: "400px", overflow: "auto" }}>
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      <div style={{ height: virtualizer.getTotalSize() }}>
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        {virtualizer.getVirtualItems().map((virtualRow) => (
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          <div
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            key={virtualRow.key}
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            style={{
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              position: "absolute",
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              top: virtualRow.start,
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              height: virtualRow.size,
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            }}
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          >
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            {items[virtualRow.index].name}
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          </div>
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        ))}
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      </div>
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    </div>
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  )
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}

Performance Measurement

Effective optimization requires continuous measurement.

Core Web Vitals Monitoring

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class PerformanceMonitor {
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  constructor() {
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    this.metrics = {}
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    this.setupObservers()
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  }
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  setupObservers() {
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    // Largest Contentful Paint
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    new PerformanceObserver((list) => {
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      const entries = list.getEntries()
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      this.metrics.lcp = entries[entries.length - 1].startTime
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    }).observe({ type: "largest-contentful-paint" })
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    // Interaction to Next Paint (replaced FID March 2024)
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    new PerformanceObserver((list) => {
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      for (const entry of list.getEntries()) {
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        if (!this.metrics.inp || entry.duration > this.metrics.inp) {
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          this.metrics.inp = entry.duration
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        }
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      }
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    }).observe({ type: "event", buffered: true, durationThreshold: 16 })
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    // Cumulative Layout Shift
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    let clsValue = 0
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    new PerformanceObserver((list) => {
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      for (const entry of list.getEntries()) {
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        if (!entry.hadRecentInput) {
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          clsValue += entry.value
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          this.metrics.cls = clsValue
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        }
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      }
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    }).observe({ type: "layout-shift" })
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  }
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  report() {
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    navigator.sendBeacon("/api/metrics", JSON.stringify(this.metrics))
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  }
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}

Long Task Detection

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const longTaskObserver = new PerformanceObserver((list) => {
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  for (const entry of list.getEntries()) {
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    if (entry.duration > 100) {
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      console.warn(`Long task: ${entry.duration.toFixed(0)}ms`, {
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        attribution: entry.attribution,
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      })
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    }
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  }
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})
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longTaskObserver.observe({ type: "longtask" })

Conclusion

JavaScript performance optimization requires understanding browser internals:

• Script loading trades off parser blocking against execution timing—use defer for order-dependent application code, async for independent scripts
• Task scheduling with scheduler.yield() maintains priority while yielding, unlike setTimeout which loses queue position
• Code splitting reduces initial load but creates chunk waterfalls—mitigate with preload hints
• Tree shaking requires ES modules and explicit side-effect declarations
• Web Workers move computation off-thread but add message serialization overhead—use transfer for large buffers

Measure before optimizing. INP, LCP, and Long Task observers provide visibility into actual user experience, not just synthetic benchmarks.

Appendix

Prerequisites

• Familiarity with JavaScript event loop and task queue model
• Understanding of browser rendering pipeline (parse, style, layout, paint)
• Basic React knowledge for framework-specific sections

Summary

• Script loading: defer = order-preserved after parsing; async = executes on arrival (no order); module = deferred with dependency resolution
• 50ms threshold: RAIL model target—100ms perceived instant, 50ms task budget leaves room for input handling
• scheduler.yield(): Chrome 129+, maintains priority via continuation queues; falls back to setTimeout(0) elsewhere
• Tree shaking: Requires ES modules (static structure); fails with side effects, dynamic imports, missing sideEffects field
• Web Workers: Off-thread computation; use transfer for large ArrayBuffers; promise rejections don’t propagate
• INP replaced FID: March 2024, measures full interaction latency not just input delay

References

Specifications

• WHATWG HTML - Scripting - Script loading attributes and execution order
• W3C Long Tasks API - 50ms threshold definition
• WICG Prioritized Task Scheduling - scheduler.yield() and scheduler.postTask()
• W3C requestIdleCallback - 50ms deadline rationale
• WHATWG HTML - Web Workers - Worker lifecycle and messaging
• WHATWG - Structured Clone - Serialization algorithm and transferables
• ECMA-262 - ES module static structure

Official Documentation

• MDN - Script Element - async, defer, module
• MDN - scheduler.yield() - API reference and browser support
• MDN - Web Workers API - Worker patterns
• MDN - Transferable Objects - Zero-copy transfer
• webpack - Code Splitting - Dynamic imports and chunk configuration
• webpack - Tree Shaking - sideEffects field
• React - lazy - Component-level code splitting
• React - Server Components - Server/client boundary

Core Maintainer Content

• WICG Yield Explainer - Priority inheritance mechanism
• Chrome Developers - scheduler.yield() - setTimeout limitations
• V8 Blog - Spectre - SharedArrayBuffer security restrictions
• React 18 Suspense SSR Architecture - Streaming and selective hydration

Industry Expert Content

• web.dev - Optimize Long Tasks - Task scheduling strategies
• web.dev - INP - Interaction to Next Paint metric
• web.dev - CommonJS Bundles - Why CommonJS prevents tree shaking