Individual skill
legacy-js
Use when auditing slow page loads, heavy assets, or rendering delays related to Avoid serving legacy JavaScript to modern browsers. Verify the actual bottleneck in DevTools, Lighthouse, or field data before recommending changes.
skills/legacy-js/SKILL.md
Source commit e8d14d049010e2ce25afcd5e8609dfcbbe63e47e
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- Matches the observed repository HEAD
- Yes
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- Yes
- Repository scan complete
- No
- Skill scan complete
- Yes
- Observed behavior
- Instructions only
- Last checked
- Nov 9, 2026
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Repository context
- Repository stars (not skill ratings)
- 74,696
- Repository last updated
- Oct 31, 2026
- License
- MIT
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- frontend Repository topic
- front-end-development Repository topic
- front-end-developer-tool Repository topic
- guidelines Repository topic
- checklist Repository topic
- reference Repository topic
- web-development Repository topic
- html Repository topic
- css Repository topic
- javascript Repository topic
- data Derived category
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SKILL.mdcore · 1,777 bytesreferences/rule.mddoc · 3,479 bytes
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# Avoid serving legacy JavaScript to modern browsers
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.
