medium
html•Jan 7, 2026
The Complete Guide to <script> Loading in Modern Web Apps: async, defer, and ES Modules
Script tags can block HTML parsing, delay interactivity, and alter event timing. The right strategy is crucial for building fast, secure, and maintainable front-ends.
devTo
Original source
Key summary
This guide explains the importance of script loading strategy in building fast, secure, and maintainable front-ends. It covers classic scripts, async, defer, and ES Modules, and provides tips on how to optimize performance and security.
TL;DR
- Script tags can block HTML parsing and delay interactivity.
- Use defer or type="module" for most scripts.
- Use async only for truly independent third-party tags.
- Prime your ESM graph with modulepreload.
- Dodge top-level await in shared modules.
- Use CSP with nonces/SRI for third-party stuff.
- Push CPU-heavy work to module workers.
- Verify wins with RUM and PerformanceObserver.
This content is automatically summarized using AI. For full context, always refer to the original article.
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