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JavaScriptAdvanced#performance#memory
JavaScriptIntermediate#browser#performance
What is the difference between defer and async on a script tag?
Both download the script without blocking parsing. async executes as soon as it is ready, in unpredictable order; defer waits until the HTML is parsed and preserves document order.
Example
<script src="analytics.js" async></script> <script src="app.js" defer></script>