Audit JavaScript delivered to modern browsers for unnecessary older code and compatibility patches. Confirm performance costs with measurements, then review build targets and browser-specific delivery to reduce avoidable download size and processing work.
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.
Check whether JavaScript delivered to modern browsers contains unnecessary ES5 code or polyfills, which provide compatibility for older environments. This skill examines output files, requests, and rendering steps to identify what increases network traffic or CPU load and explain how to measure those costs. It also considers module/nomodule delivery, which serves different code according to the browser, and changing build targets to newer environments.
Use it when pages load slowly or when you need to understand why so much JavaScript is being delivered. It also supports reviews of settings that transform code for older browsers, using actual measurements to decide whether changes are needed.
Before recommending changes, confirm the bottleneck using DevTools, Lighthouse, or data from actual usage environments. Do not attribute slow performance to the code format alone. Consider delivery methods in light of the browsers that need to be supported.
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.
Modern browsers can execute ES6+ code faster and more efficiently; serving them transpiled ES5 with polyfills adds unnecessary weight.
Analyze the project's JavaScript output to see if modern browsers are being served unnecessary ES5 code and polyfills.
Update the build configuration to use differential serving and set a modern target for your primary JavaScript output.
Explain how serving ES6+ code to modern browsers improves performance and why transpiling everything to ES5 is no longer recommended.
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/rules/performance/legacy-js
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