The answer
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.
async vs defer Script Loading in JavaScript
When should you use async in JavaScript?
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 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.
No. async scripts execute in whatever order they finish downloading. If script A is before script B but B downloads first, B executes first. Use defer if order matters.
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