Does async guarantee script execution order?
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.
Explore This Question
Search for additional explanations using these tools:
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.
Still have questions?
Browse all our FAQs or reach out to our support team