Which is better for page performance: async or defer?
defer is better for most cases. It does not block parsing or rendering and guarantees execution order. async is only better for independent scripts (analytics, ads) where order does not matter.
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More FAQs in async and defer Interview Questions (Ola)
async downloads in parallel and executes as soon as ready (unordered, briefly blocks parsing). defer downloads in parallel and executes after HTML parsing (ordered, non-blocking).
No. async scripts execute in whatever order they finish downloading. If a later script downloads first, it executes first. Use defer if order matters.
Yes. defer scripts execute in the order they appear in the HTML, after all HTML parsing is complete. This makes defer safe for scripts with dependencies.
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