Avoid serving legacy JavaScript to modern browsers
SkillMedialegacy-js is a skill that guides an AI agent through diagnosing whether old JavaScript files are slowing down web pages. It is used when auditing slow page loads, heavy assets, or rendering delays related to Avoid serving legacy JavaScript to modern browsers. It also directs the agent to verify the actual bottleneck in DevTools, Lighthouse, or field data before recommending changes.
Available today. Use it from your connected AI after setup.
No other account needed.
Have an agent that can load skills.
Then ask your AI: use the Avoid serving legacy JavaScript to modern browsers skill
What your AI can do with it
- Diagnose whether old JavaScript files are slowing down web pages
- Audit slow page loads, heavy assets, and rendering delays
- Address issues related to Avoid serving legacy JavaScript to modern browsers
- Verify the actual bottleneck in DevTools, Lighthouse, or field data
- Ensure bottleneck verification happens before recommending changes
Getting started
- Have an agent that can load skills.
- Add the legacy-js skill to the agent's available skills.
- Ask the agent to audit a page for slow loads, heavy assets, or rendering delays.
- Have access to DevTools, Lighthouse, or field data so the agent can verify the actual bottleneck before changes are recommended.
What this skill tells your AI
The instructions your AI receives, as published by thedaviddias/front-end-checklist in skills/legacy-js/SKILL.md and read by ahel’s review.
Modern browsers can execute ES6+ code faster and more efficiently; serving them transpiled ES5 with polyfills adds unnecessary weight.
Quick Reference
- Use differential serving (module/nomodule) to target modern and legacy browsers
- Avoid heavy ES5 polyfills for browsers that support ES6+
- Compile code to modern targets to reduce bundle size and improve execution speed
Check
Analyze the project's JavaScript output to see if modern browsers are being served unnecessary ES5 code and polyfills.
Fix
Update the build configuration to use differential serving and set a modern target for your primary JavaScript output.
Explain
Explain how serving ES6+ code to modern browsers improves performance and why transpiling everything to ES5 is no longer recommended.
Code Review
Review the routes, assets, and loading behavior that affect Avoid serving legacy JavaScript to modern browsers. 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/en/rules/performance/legacy-js
Signals
- GitHub stars
- 74k
- Forks
- 7k
- Last commit
- Aug 2026
Questions
- When should this skill be used?
- Use it when auditing slow page loads, heavy assets, or rendering delays related to Avoid serving legacy JavaScript to modern browsers.
- Does it recommend changes right away?
- No. It verifies the actual bottleneck in DevTools, Lighthouse, or field data before recommending changes.
- What does the skill do?
- It guides your agent through diagnosing whether old JavaScript files are slowing down web pages.
Advanced
- Item type
- skill
- Key
legacy-js- Source
- github.com/thedaviddias/front-end-checklist