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Get Started Free →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.
.claude/skills/thedaviddias-legacy-js/SKILL.md| Test case | Without → With | Effect | Δ tokens | Δ turns |
|---|---|---|---|---|
| case-19 | ✗→✓ | ▲ Improved | -9% | 0% |
| case-01 | ✓→✓ | ＝ Same ✓ | -15% | 0% |
| case-02 | ✓→✓ | ＝ Same ✓ | 15% | 0% |
| case-03 | ✓→✓ | ＝ Same ✓ | -5% | 0% |
| case-04 | ✓→✓ | ＝ Same ✓ | -23% | 0% |
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/en/rules/performance/legacy-js
| Case | Status | Duration (ms) | Turns | Tokens | Tool calls | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Without | With | Δ | Without | With | Δ | Without | With | Δ | Without | With | Δ | ||
case-01 | pass→pass | 16,496 | 12,162 | -26% | 1 | 1 | 0% | 2,889 | 2,456 | -15% | 0 | 0 | — |
case-02 | pass→pass | 10,812 | 11,059 | +2% | 1 | 1 | 0% | 2,060 | 2,361 | +15% | 0 | 0 | — |
case-03 | pass→pass | 13,378 | 10,956 | -18% | 1 | 1 | 0% | 2,474 | 2,354 | -5% | 0 | 0 | — |
case-04 | pass→pass | 5,525 | 3,158 | -43% | 1 | 1 | 0% | 1,145 | 884 | -23% | 0 | 0 | — |
case-05 | pass→pass | 13,380 | 10,224 | -24% | 1 | 1 | 0% | 2,390 | 2,049 | -14% | 0 | 0 | — |
case-06 | pass→pass | 12,063 | 12,192 | +1% | 1 | 1 | 0% | 2,055 | 2,365 | +15% | 0 | 0 | — |
case-07 | pass→pass | 15,373 | 10,895 | -29% | 1 | 1 | 0% | 2,599 | 2,057 | -21% | 0 | 0 | — |
case-08 | pass→pass | 15,197 | 12,281 | -19% | 1 | 1 | 0% | 2,688 | 2,484 | -8% | 0 | 0 | — |
case-09 | pass→pass | 10,286 | 7,488 | -27% | 1 | 1 | 0% | 1,931 | 1,644 | -15% | 0 | 0 | — |
case-10 | pass→pass | 12,293 | 6,439 | -48% | 1 | 1 | 0% | 2,138 | 1,532 | -28% | 0 | 0 | — |
case-11 | pass→pass | 12,231 | 13,639 | +12% | 1 | 1 | 0% | 2,270 | 2,816 | +24% | 0 | 0 | — |
case-12 | pass→pass | 14,651 | 9,759 | -33% | 1 | 1 | 0% | 2,417 | 1,793 | -26% | 0 | 0 | — |
case-13 | pass→pass | 18,898 | 15,147 | -20% | 1 | 1 | 0% | 3,147 | 2,846 | -10% | 0 | 0 | — |
case-14 | pass→pass | 15,386 | 11,073 | -28% | 1 | 1 | 0% | 2,623 | 2,282 | -13% | 0 | 0 | — |
case-15 | pass→pass | 10,280 | 4,211 | -59% | 1 | 1 | 0% | 2,028 | 1,029 | -49% | 0 | 0 | — |
case-16 | pass→pass | 10,047 | 6,794 | -32% | 1 | 1 | 0% | 1,860 | 1,558 | -16% | 0 | 0 | — |
case-17 | pass→pass | 10,103 | 6,854 | -32% | 1 | 1 | 0% | 1,843 | 1,490 | -19% | 0 | 0 | — |
case-18 | pass→pass | 14,498 | 10,567 | -27% | 1 | 1 | 0% | 2,491 | 2,230 | -10% | 0 | 0 | — |
case-19 | fail→pass | 18,232 | 13,227 | -27% | 1 | 1 | 0% | 3,075 | 2,785 | -9% | 0 | 0 | — |
case-20 | pass→pass | 12,777 | 10,473 | -18% | 1 | 1 | 0% | 2,484 | 2,415 | -3% | 0 | 0 | — |
case-21 | pass→pass | 17,165 | 11,422 | -33% | 1 | 1 | 0% | 3,391 | 2,684 | -21% | 0 | 0 | — |
case-22 | pass→pass | 10,588 | 10,128 | -4% | 1 | 1 | 0% | 2,229 | 2,299 | +3% | 0 | 0 | — |
DecimalAI ran this skill against gemini-3.6-flash twice over the same eval suite — once with the skill loaded and once without — and compared the two runs case by case. 22 cases were attempted. The headline lift of +5 percentage points is the difference between those two pass rates over the 22 comparable cases.
Without the skill loaded, the model failed this case. With it loaded, the same prompt on the same model passed. This is one improved case from the latest verified run; every case, including any that regressed, is in the table above.
Other measured skills in the registry, with their headline benchmark lift.