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Async Script Tester

Test JavaScript loading strategies (async, defer, module) in a real-time sandbox. Analyze blocking behavior, execution order, and network timing.

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About

This tool analyzes the critical rendering path impact of external JavaScript resources. By isolating execution in a controlled sandbox, it measures the delta between Tfetch (network retrieval) and Texec (runtime parsing). Inefficient script loading is a primary cause of LCP (Largest Contentful Paint) degradation.

We simulate the browser's parser behavior using real DOM injection. Unlike static linters, this tool executes the code to detect runtime errors, CORS violations, and main-thread blocking events. Use this to validate async vs defer strategies before deploying to production.

javascript performance async defer web-vitals

Formulas

The impact on the main thread is calculated by measuring the delay in the event loop during script execution.

Tblocking ≈ n∑i=1 (tframe_actual − tframe_budget)

Where tframe_budget is typically 16.67ms (60fps). If Tload happens during HTML parsing without defer, the parser halts:

DOMready = tparse + tscript_exec

Reference Data

AttributeFetch BehaviorExecution TimingBlocking Risk
<script>Synchronous (Pauses HTML Parser)Immediate (Blocks Rendering)HIGH
<script async>Parallel (Background)Immediately upon load (Interrupts Parser)MEDIUM
<script defer>Parallel (Background)After HTML Parsing (Before DOMContentLoaded)LOW
type="module"Parallel (Background)Deferred by defaultLOW

Frequently Asked Questions

If the remote server does not provide `Access-Control-Allow-Origin` headers, browsers block the resource from being read by the testing sandbox. This simulates exactly what would happen on your site.
Async scripts execute as soon as they finish downloading, potentially pausing the HTML parser. Defer scripts wait until the entire HTML document is parsed before executing, preserving the render sequence.
Yes. Select "Raw Code" input mode. The tool will convert your code into a Blob URL to simulate an external network request, allowing for accurate timing analysis.
Modern JavaScript engines are extremely fast. For small scripts, execution might take less than 1ms. Additionally, due to browser security (Spectre mitigations), high-resolution timers may have reduced precision.