Chrome

Chrome DevTools Protocol, extension Manifest V3, and debugging patterns that prevent common automation failures.

Chrome DevTools Protocol (CDP)

Get tab WebSocket URL first: Never connect to ws://localhost:9222/devtools/browser directly. Fetch http://localhost:9222/json/list and use webSocketDebuggerUrl from the active tab.

Enable domains before use: Runtime.enable and Page.enable must be called before any Runtime.evaluate or Page.navigate commands.

CDP is async: Wait for response before sending next command. Use Promise-based wrapper with response ID tracking.

Screenshot on high-DPI: Include fromSurface: true and scale: 2 in Page.captureScreenshot params for Retina displays.

Get response body separately: Network.responseReceived doesn't include body. Call Network.getResponseBody with requestId after response completes.

Chrome Extension Manifest V3

Permissions split: Use permissions for APIs, host_permissions for URLs. Never use http://*/* in permissions.

Service workers terminate: No persistent state. Use chrome.storage.local instead of global variables. Use chrome.alarms instead of setInterval.

Content script isolation: Can't access page globals. Use chrome.scripting.executeScript with func for page context. Use window.postMessage for content↔page communication.

Storage is async: chrome.storage.local.get() returns Promise, not data. Always await. Handle QUOTA_EXCEEDED errors.

Context Detection

Detect actual Chrome (not Edge/Brave): Check window.chrome && navigator.vendor === "Google Inc." and exclude Opera/Edge.

Extension context types:

• chrome.runtime.id exists → content script
• chrome.runtime.getManifest exists → popup/background/options
• chrome.loadTimes exists but no runtime → regular Chrome web page

Manifest version check: Wrap chrome.runtime.getManifest() in try-catch. Use chrome.action for V3, chrome.browserAction for V2.

Performance Debugging

Memory API conditional: Check 'memory' in performance before accessing performance.memory.usedJSHeapSize.

Use performance marks: performance.mark() and performance.measure() for sub-frame timing. Clear marks to prevent memory leaks.

Layout thrash detection: PerformanceObserver with entryTypes: ['measure', 'paint', 'largest-contentful-paint']. Flag entries >16.67ms.

Network Debugging

Block before navigate: Call Network.setBlockedURLs before Page.navigate, not after.

Request interception: Use Network.setRequestInterception with requestStage: 'Request' for granular control. Return errorReason: 'BlockedByClient' to block.

Security Contexts

Mixed content: HTTPS pages can't load HTTP resources. Check location.protocol vs resource URL.

CORS errors: TypeError on cross-origin fetch usually means CORS. Check DevTools Network tab for specific error.

Secure context required: File System Access API, Clipboard API require window.isSecureContext === true and user gesture.