How to Fix Every JavaScript Error
The JavaScript Checker runs your scripts, reports console errors and warnings, flags render-blocking patterns, measures bundle size and parse time, and tests how Googlebot's headless Chromium sees your page. JS errors silently break features, hurt Core Web Vitals, and prevent crawler indexing. This index covers every category of finding.
By error type
Pick the category matching your finding:
⚠️ Fix JavaScript syntax errors LIVE
A syntax error stops the entire script. Missing semicolons in minified bundles, unterminated strings, mismatched braces from auto-merge conflicts. The diagnostic workflow: source map → original file → root cause.
🔴 Fix console errors at page load LIVE
"Uncaught TypeError: x is null", "ReferenceError: y is not defined", 404s for missing JS files, CORS errors from third-party scripts. How to prioritise: errors first, warnings later, third-party last (often unfixable).
🚦 Fix render-blocking scripts LIVE
A <script src> in <head> without async or defer halts page render. Add defer for scripts that need DOM, async for independent ones (analytics), inline only critical-path. The impact on Largest Contentful Paint.
📦 Fix oversized JS bundles LIVE
A 2MB main bundle costs 10+ seconds on 3G. Code splitting by route, lazy-loading components, tree-shaking unused exports, replacing heavyweight libraries (Moment → date-fns, Lodash → individual imports). Realistic targets: under 200KB compressed.
🏷️ Fix third-party tag bloat LIVE
Marketing/analytics/heatmap/chat scripts compound: each loads its own dependencies. The tag-management approach via GTM, server-side tagging for analytics, and the "does this tag still earn its bytes?" quarterly audit.
🤖 Fix JavaScript that crawlers can't render LIVE
Googlebot renders JS in two waves; AI crawlers usually skip JS entirely. Content that appears only after JS runs may be invisible to AI engines. Fix with SSR, SSG, or dynamic rendering for bot user agents.
⏱️ Fix long tasks blocking the main thread LIVE
Tasks over 50ms block user interactions, hurting Interaction to Next Paint (INP). Break long tasks into chunks with requestIdleCallback or setTimeout, move heavy computation to Web Workers, virtualise long lists.
💧 Fix JavaScript memory leaks LIVE
SPAs with route changes that don't tear down listeners, observers, intervals. Memory climbs until the tab crashes. The Chrome DevTools Memory profiler workflow, the common React/Vue/Svelte patterns that leak, and how to set up CI guards.
By framework
Common JS findings vary by stack:
⚛️ Fix JS issues in React / Next.js PLANNED
Hydration mismatches, useEffect cleanup, the bundle-splitting patterns Next.js provides, and the SSR-vs-CSR decision for each page type.
📰 Fix JS issues in WordPress PLANNED
Theme/plugin script conflicts, jQuery dependency hell, deferred-loading patterns that don't break Elementor/Divi, the WP-Rocket vs Perfmatters trade-off.
🛒 Fix JS issues in Shopify PLANNED
Theme script optimisation, app-embedded scripts that destroy performance, the move to Hydrogen for high-traffic stores.
What our JS Checker tests
The checker loads your page in headless Chromium, captures every console error and warning, measures bundle size and parse/execute time, identifies render-blocking patterns, lists third-party tags with their byte and time cost, and replicates how Googlebot sees the rendered DOM. For the full reference, see the JS Checker Guide or sample report.
🔧 Audit your scripts first
Run the checker. Most sites have 3-5 specific scripts costing them seconds of load time — kill those and pages feel instantly faster.
Run JS Checker →