Inspect compressed JavaScript bundles, the files that package application code, to find excessive size and loading costs. Recommend splitting code, removing unused portions, and reviewing dependencies based on measured effects on page responsiveness.
Use when auditing slow page loads, heavy assets, or rendering delays related to Optimize JavaScript bundle size. Verify the actual bottleneck in DevTools, Lighthouse, or field data before recommending changes.
js-file-size examines JavaScript files bundled for delivery to identify oversized files and loading costs. It checks files larger than 200KB after Gzip/Brotli compression and works toward reducing their size through code splitting, removal of unused code, and dependency review.
Use it when a page is visible but takes time to respond to interaction, or when it runs slowly on less powerful devices. It explains how large JavaScript files affect parsing time, execution time, and data transfer. It also identifies specific files, requests, and rendering steps that create unnecessary work.
The 200KB threshold is the size guideline this skill uses for its checks. Before recommending improvements, it confirms the cause with DevTools, Lighthouse, or data from real usage. Findings are intended to include the measurement method used to verify each problem.
This overview was written by AI (OpenAI Codex) from the public SKILL.md. Check the original below for the exact behavior.
AI rated this skill on four points using only the contents of its SKILL.md. Stars and popularity are not taken into account.
The total counts each point as A = 25, B = 15, C = 5. Rated by AI (OpenAI Codex) as of Oct 23, 2026. Treat it as a guide and read the original before installing.
Review the instructions the agent will receive before you install.
Excessive JavaScript increases parse and execution time, especially on low-end devices, leading to sluggish user experiences and high data costs.
Review the JavaScript bundle sizes and identify any files exceeding 200KB (Gzip/Brotli).
Implement code-splitting, tree-shaking, and dependency audits to reduce the size of the JavaScript bundles.
Explain how large JavaScript files impact the main thread and delay user interactivity (FID/TTI).
Review the routes, assets, and loading behavior that affect Optimize JavaScript bundle size. Flag exact files, requests, or rendering steps that add unnecessary network, CPU, or layout cost, and describe the measurement method used to confirm the issue.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
Rule page: https://frontendchecklist.io/rules/performance/js-file-size
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Create and review custom 404 pages that help visitors recover from broken or moved links through search and navigation. Check the rendered page and HTTP response so missing content returns a proper 404 status.
Audit or create a dedicated About page that explains a website’s purpose and identifies its people or organization. Review the rendered output and supporting information, including expertise and contact details, to find gaps.
Review and add automated accessibility checks for components, pages, and continuous integration workflows. Use axe-based tools to detect structural and attribute issues, while identifying checks that still need keyboard or screen reader testing.
Review sign-in, registration, multi-factor authentication, and account recovery for accessibility barriers. Find blocked paste, difficult code entry, and inaccessible backup methods, then suggest authentication paths that support password managers, autofill, or device approval.
Review form errors, success messages, and live updates so screen reader users receive important notifications. Check the HTML delivered to the browser, announcement settings, display duration, and dismissal controls, then identify specific elements that need changes.
Review HTML tables so screen reader users can understand how headers and data relate. Check header markup, table descriptions, and merged cells, then identify specific fixes and ways to verify them.