I Optimize
boussadjra/vuestrata
Improve interface performance across loading speed, rendering, animations, images, and bundle size.
A skill your agent uses when auditing slow page loads, heavy assets, or rendering delays related to Avoid serving legacy JavaScript to modern browsers.
Install Legacy JS
$ npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install thedaviddias/Front-End-Checklist legacy-js --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/legacy-js .claude/skills/legacy-js && rm -rf skills-srcUse ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.
Claude Code skills documentation · loads skills from .claude/skills/
Install the "legacy-js" agent skill from https://github.com/thedaviddias/Front-End-Checklist/tree/main/skills/legacy-js into .claude/skills/legacy-js/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "legacy-js", then confirm the skill loads.Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$skill-installer install https://github.com/thedaviddias/Front-End-Checklist/tree/main/skills/legacy-jsType this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.
$ npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install thedaviddias/Front-End-Checklist legacy-js --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .agents/skills && cp -r skills-src/skills/legacy-js .agents/skills/legacy-js && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "legacy-js" agent skill from https://github.com/thedaviddias/Front-End-Checklist/tree/main/skills/legacy-js into .agents/skills/legacy-js/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "legacy-js", then confirm the skill loads.Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install thedaviddias/Front-End-Checklist legacy-js --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/skills/legacy-js .cursor/skills/legacy-js && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "legacy-js" agent skill from https://github.com/thedaviddias/Front-End-Checklist/tree/main/skills/legacy-js into .cursor/skills/legacy-js/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "legacy-js", then confirm the skill loads.Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gemini skills install https://github.com/thedaviddias/Front-End-Checklist.git --path skills/legacy-js--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install thedaviddias/Front-End-Checklist legacy-js --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/skills/legacy-js .gemini/skills/legacy-js && rm -rf skills-srcUse ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "legacy-js" agent skill from https://github.com/thedaviddias/Front-End-Checklist/tree/main/skills/legacy-js into .gemini/skills/legacy-js/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "legacy-js", then confirm the skill loads.Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gh skill install thedaviddias/Front-End-Checklist legacy-jsInstalls for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).
$ npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .github/skills && cp -r skills-src/skills/legacy-js .github/skills/legacy-js && rm -rf skills-srcUse ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "legacy-js" agent skill from https://github.com/thedaviddias/Front-End-Checklist/tree/main/skills/legacy-js into .github/skills/legacy-js/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "legacy-js", then confirm the skill loads.GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install thedaviddias/Front-End-Checklist legacy-js --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/thedaviddias/Front-End-Checklist.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/skills/legacy-js .opencode/skills/legacy-js && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "legacy-js" agent skill from https://github.com/thedaviddias/Front-End-Checklist/tree/main/skills/legacy-js into .opencode/skills/legacy-js/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "legacy-js", then confirm the skill loads.OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
Facts
- Skill name
legacy-js- GitHub stars
- 74k
- Token cost
- ~445 tokens
- SKILL.md length
- 176 words
- Files
- 2 (incl. references)
- Skills in repo
- 390
- Repo updated
- First seen
- Licence
- MIT
At a glance
A skill your agent uses when auditing slow page loads, heavy assets, or rendering delays related to Avoid serving legacy JavaScript to modern browsers.
- Auditing slow page loads
- SKILL.md covers Quick Reference, Check, Fix and Explain, plus 1 more section
- Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
- Rendering delays related to Avoid serving legacy JavaScript to modern browsers
What it does
Legacy JS is an agent skill from thedaviddias/Front-End-Checklist. 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.
Its SKILL.md is about 450 tokens, which your agent loads only when the skill is triggered. The skill folder holds 2 other files, including reference files (for example `references/rule.md`).
It sits in Frontend & Design, covering Web performance. It works with JavaScript. The repository describes itself as: 🗂 The essential checklist for modern web development, for humans and AI agents. The licence is MIT.
When your agent uses it
- Auditing slow page loads
- Rendering delays related to Avoid serving legacy JavaScript to modern browsers
Example prompts
- “/legacy-js”
What it can do on your machine
Read from SKILL.md and the folder at commit e8d14d0. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md.
From the folder's file list and the shell code blocks in SKILL.md.
Network
Links to these hosts (documentation or services it may open):
frontendchecklist.io
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.
Context cost
Legacy JS loads about 445 tokens when it runs, and up to ~1.3k if it reads all its reference files. Until then it costs about 60 tokens; SKILL.md has 176 words of instructions outside code blocks.
Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.
Safety
Auto-check passed
The automated check found no risky patterns in SKILL.md.
Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.
SKILL.md
The full file from thedaviddias/Front-End-Checklist at commit e8d14d0, republished under its MIT licence (© thedaviddias). 176 words, ~445 tokens.
Download SKILL.mdSave it as
.claude/skills/legacy-js/SKILL.md (or your agent's skills folder). This skill also uses 1 other file; get the full folder from GitHub.- name
- legacy-js
- description
- 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.
- metadata.category
- performance
- metadata.priority
- medium
- metadata.difficulty
- intermediate
- metadata.estimatedTime
- 10
- metadata.source
- frontendchecklist.io
- metadata.url
- https://frontendchecklist.io/rules/performance/legacy-js
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.
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/rules/performance/legacy-js
© thedaviddias, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
SKILL.md and 1 other file (references) in skills/legacy-js of thedaviddias/Front-End-Checklist.
- SKILL.md
- references/rule.md
Open the folder on GitHubat commit e8d14d0
Compare with similar skills
Legacy JS next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Legacy JS this skillthedaviddias/Front-End-Checklist | 74k | — | ~445 | Automated safety check: Pass | MIT | |
| I Optimizeboussadjra/vuestrata | 111 | 11 repos | ~1.9k | Automated safety check: Pass | MIT | |
| Compare Array Bundle SizePostHog/posthog-js | 634 | — | ~599 | Automated safety check: Pass | Custom licence | |
| Web Performancemozilla/firefox-devtools-mcp | 478 | — | ~1.2k | Automated safety check: Pass | Custom licence | |
| Igniteui Wc Integrate With FrameworkIgniteUI/igniteui-webcomponents | 170 | — | ~1.4k | Automated safety check: Pass | MIT | |
| Web Performance Optimizationdavila7/claude-code-templates | 33k | 7 repos | ~4k | Automated safety check: Pass | MIT |
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Compare Array Bundle Size
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Web Performance
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Find and fix why a website is slow by capturing and analyzing a Firefox performance profile, or by analyzing a profile the user already has (a saved file or a profiler.firefox.com share link).
Igniteui Wc Integrate With Framework
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npm Package Comparator
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Compare two or more npm packages side by side using live data — downloads, bundle size, GitHub stars, last update, known vulnerabilities, and community sentiment.
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Related
Works with
Categories
Questions about Legacy JS
What does Legacy JS do?
A skill your agent uses when auditing slow page loads, heavy assets, or rendering delays related to Avoid serving legacy JavaScript to modern browsers. Legacy JS is an agent skill from thedaviddias/Front-End-Checklist. Use when auditing slow page loads, heavy assets, or rendering delays related to Avoid serving legacy JavaScript to modern browsers.
When should I use Legacy JS?
Legacy JS fits situations like: auditing slow page loads; rendering delays related to Avoid serving legacy JavaScript to modern browsers.
How do I install Legacy JS in Claude Code?
Run `npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a claude-code`. Or copy the skill folder (skills/legacy-js in thedaviddias/Front-End-Checklist) into .claude/skills/legacy-js in your project. Claude Code loads it when a task matches its description.
How do I install Legacy JS in Codex?
Run `npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a codex`. Or copy the skill folder (skills/legacy-js in thedaviddias/Front-End-Checklist) into .agents/skills/legacy-js in your project. Codex loads it when a task matches its description.
Can I use Legacy JS in Cursor, Gemini CLI or GitHub Copilot?
Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add thedaviddias/Front-End-Checklist --skill legacy-js -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/legacy-js, .gemini/skills/legacy-js, .github/skills/legacy-js and .opencode/skills/legacy-js in your project.
What does Legacy JS need to run?
SKILL.md names no scripts, command-line tools or credentials: Legacy JS is instructions for the agent only.
Does Legacy JS access the network?
SKILL.md names 1 domain. As links in the text: frontendchecklist.io. This is read from the text; nothing was executed.
Is Legacy JS safe to install?
Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.
What licence does Legacy JS use?
Legacy JS is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Legacy JS use?
About 445 tokens (SKILL.md is roughly 1.8k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 870 tokens, read only when the agent opens those files.
What are the alternatives to Legacy JS?
Skills that share tags, products or a category with Legacy JS: I Optimize (boussadjra/vuestrata, 111 stars), Compare Array Bundle Size (PostHog/posthog-js, 634 stars), Web Performance (mozilla/firefox-devtools-mcp, 478 stars) and Igniteui Wc Integrate With Framework (IgniteUI/igniteui-webcomponents, 170 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Legacy JS?
thedaviddias (a GitHub user) maintains it in thedaviddias/Front-End-Checklist, which has 74,421 GitHub stars. The repository holds 390 skills in this directory. The repository was last updated on October 6, 2026.
Source: thedaviddias/Front-End-Checklist on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.