Loading multiple versions of the same library (like two versions of jQuery or Lodash) is a common cause of unnecessary page weight and potential runtime errors.

Code Examples#

1. Checking for Duplicates with npm/pnpm#

Use your package manager to find multiple versions of a dependency.

# For npm
npm ls lodash
 
# For pnpm
pnpm why lodash

2. Consolidating with Package Overrides#

If different sub-dependencies require different versions, you can sometimes force a single version.

// package.json
{
  "overrides": {
    "lodash": "^4.17.21"
  }
}

3. Identifying Duplicates in the Browser#

You can check for multiple global variables in the console.

// Check for multiple jQuery versions
console.log('jQuery version 1:', window.jQuery?.fn?.jquery);
// Some scripts might alias jQuery

Why It Matters#

• Bundle Bloat: Each duplicate library adds to the total amount of JavaScript the browser must download, parse, and execute.
• Memory Overhead: Multiple instances of a library consume more memory, which can impact performance on low-end devices.
• Version Conflicts: Different versions of a library may have incompatible APIs or global state, leading to hard-to-debug bugs.
• Execution Time: The browser spends more time in the "Compile Script" and "Evaluate Script" phases of the rendering process.

Best Practices#

Start with Bundlephobia (opens in a new tab) or your local bundle analyzer before deduping packages, because the real problem is usually one duplicated heavyweight dependency rather than every shared utility in the tree.

✅ Analyze Bundles: Use tools like webpack-bundle-analyzer or rollup-plugin-visualizer to see exactly what's in your bundle. ✅ Use Peer Dependencies: If you are a library author, use peerDependencies to avoid forcing users to install duplicate versions. ✅ Audit Third-Party Tags: Use Google Tag Manager or a similar tool to audit scripts that might be loading their own dependencies. ✅ Dependency Deduplication: Regularly run npm dedupe or pnpm dedupe to clean up your lockfile.

❌ Don't Ignore Warnings: Pay attention to "multiple versions of X" warnings during your build process. ❌ Avoid Multiple Global Libraries: Don't load libraries via CDN if they are already included in your main application bundle.

Tools & Validation#

• Bundlephobia (opens in a new tab): Check the cost of adding a new dependency.
• Webpack Bundle Analyzer (opens in a new tab): Visualizes the size of webpack output files with an interactive zoomable treemap.
• Lighthouse (opens in a new tab): Specifically flags "duplicate modules in JavaScript bundles."
• JSHint (opens in a new tab): Can help identify global variable collisions.

Standards#

• Use web.dev: Learn Performance as the standard for measuring the final production behavior, not just local synthetic output.
• Use Chrome Developers: Lighthouse overview as the standard for measuring the final production behavior, not just local synthetic output.

Verification#

Automated Checks#

• Measure the affected page or flow in Lighthouse, PageSpeed Insights, or DevTools and confirm the targeted metric improves.
• Inspect the network waterfall or performance timeline to confirm the intended resource or execution change actually took effect.

Manual Checks#

• Verify the change on a throttled mobile profile, not just local desktop.
• If this rule maps to a budget or Web Vital, confirm the page now stays within that threshold.