Does defer guarantee script execution order?
Yes. defer scripts execute in the order they appear in the HTML, after parsing completes. This makes defer safe for scripts with dependencies.
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More FAQs in async vs defer Script Loading in JavaScript
async downloads in parallel but executes as soon as ready (unordered, briefly blocks parsing). defer downloads in parallel and executes after HTML parsing (ordered, does not block).
For independent scripts that do not depend on other scripts or the DOM: analytics, ads, tracking pixels. async executes as soon as downloaded, regardless of order.
For most scripts: app logic, dependencies, scripts that need the DOM. defer downloads in parallel, executes after HTML parsing in order. It does not block rendering.
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