Milten.ioMilten

JS Scanner

Comprehensive JavaScript performance, coverage, and error analysis

Advantages

Total Blocking Time analysis
Measure how long the main thread is blocked by JavaScript. Identify long tasks that degrade interactivity and increase Time to Interactive.
Unused code detection
Per-file breakdown of unused JavaScript with exact byte counts. Quickly find dead code and oversized bundles that slow down page load.
Script loading strategy audit
See which scripts are render-blocking vs async or deferred. Spot third-party scripts that delay First Contentful Paint.
Runtime error & deprecation tracking
Capture console errors, uncaught exceptions with stack traces, and deprecated browser API usage — all in a single scan.

How it works

Tools

Validate HTML
Accessibility
JS Scanner
CSS
Design tokens

FAQ

TBT measures the total time the main thread is blocked by long JavaScript tasks (over 50 ms each). A high TBT means the page feels sluggish because user interactions are delayed until scripts finish executing.

The tool uses Chrome's built-in code coverage API to track which bytes of each script file actually execute during page load. Anything not executed is flagged as unused, shown as a percentage per file.

A script loaded without async or defer attributes blocks the HTML parser until it downloads and executes. This delays rendering. The audit flags every such script so you can add async/defer or move it to the bottom of the page.

Yes. Each script is classified as first-party or third-party. You can see which external scripts are blocking rendering, how much unused code they contribute, and whether they trigger errors or deprecated API calls.

If your scripts include source maps, the tool parses them to build a visual treemap of every module inside each bundle — showing exact sizes and code coverage so you can pinpoint which dependencies to optimize or remove.
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