Effect of unused JS in bundle on Script Evaluation during Lighthouse

Unused JavaScript in a bundle refers to code that is present in the final JavaScript files delivered to browsers but is not actually required to run your web application. This increases the overall bundle size, which leads to a direct impact on script evaluation times—a key metric measured by Lighthouse during web performance audits[1][5].

Core Effect on Script Evaluation:

• Larger Bundles Take Longer to Parse and Execute: When unused JavaScript is included in the main bundle, the browser must download, parse, and execute all of it, even if only a portion is used by the page[3][1][5]. This causes increased script evaluation time, which Lighthouse reports as part of its JavaScript performance diagnostics[3].

Technical Mechanisms:

• Script Evaluation Stages:

  1. Download: The browser fetches the JavaScript bundle.
  2. Parsing: The JavaScript engine parses the entire bundle to understand its structure and syntax.
  3. Execution: The engine executes the code, registering functions and executing top-level code.

• Unused JS Still Consumes Parsing & Evaluating Resources: Even unused code (such as obsolete functions or unused dependencies) must be parsed and evaluated, unless effective dead code elimination (tree shaking) is performed at build time[1][4].

Lighthouse Assessment:

• Lighthouse Treemap & Coverage: Lighthouse and related tools (such as the Lighthouse Treemap) visualize not just the size of each JavaScript chunk, but also what percentage of each file is unused[2]. This helps identify inefficiencies—large bundles with high unused code percentages are flagged, and their contribution to script evaluation time is highlighted.
• Evaluated in Field Performance Metrics: Lighthouse connects large, inefficient bundles to:
  • Longer main-thread blocking: Parsing and evaluating unnecessary code keeps the browser’s main thread busy, delaying interactivity[3].
  • Lower Time to Interactive (TTI) and higher Total Blocking Time (TBT): Users wait longer before they can meaningfully interact with the page[3].

Sources of Unused JS in Bundles:

• Legacy Code: Functions, variables, or whole modules left from earlier development cycles but no longer used[1][5].
• Unused Dependencies: Third-party libraries or parts thereof, imported into bundles but never utilized by the application code. Some third-party packages ship their entire codebase, not just the part you use, unless build tools correctly tree-shake the bundle[1][4].
• Conditional/Feature Branches: Code that executes only under certain conditions or on certain devices; unused branches count as unused code depending on the test scenario (e.g., desktop vs. mobile in Lighthouse)[4].

Performance Implications:

• Long Tasks: Scripts with a lot of unused code increase the likelihood of “long tasks”—blocks of scripting or evaluation that lock up the main thread for more than 50ms. These degrade user experience, especially on lower-end devices[3].
• Wasted Bandwidth and Caching Issues: Unused code increases bundle size, leading to longer downloads and poorer cache efficiency[3].

Detection and Optimization:

• Coverage Tools: Chrome DevTools Coverage tab and Lighthouse Treemap help diagnose unused JS at both the file and module level, showing which parts of bundles are never called during a typical user session[2].
• Build Optimizations: Proper configuration of bundlers (like Webpack or Rollup) and application of tree-shaking (removing unused exports) are essential to reduce unused JS and improve script evaluation times[4].

Best Practices to Mitigate the Impact:

• Aggressively remove or split code not needed for the initial render.
• Use dynamic imports or code-splitting to load only what’s needed, when it’s needed.
• Regularly audit dependencies and prune unused packages or exports[1][5].

Summary Table: Unused JS Effect on Script Evaluation (Lighthouse Context)

Aspect	Impact of Unused JS in Bundle
Bundle download time	Increases (larger bundles)
Parse/Evaluation time	Increases (all code evaluated)
Main-thread blocking	Increased (delays TTI, higher TBT)
User experience	Degraded (slower interactions)
Lighthouse score	Lowered due to inefficient scripting

In conclusion, unused JavaScript in your bundle leads to longer script evaluation times during Lighthouse analysis, decreases Lighthouse performance scores, and degrades user experience, particularly on slower devices or networks. Regularly identifying and eliminating unused code is crucial for optimal script evaluation and performant web applications[3][1][2][5].