That happens if scripts are small or cached, and the document is long enough.ĭocument order (as they go in the document).Įxecute after the document is loaded and parsed (they wait if needed), right before DOMContentLoaded. May load and execute while the document has not yet been fully downloaded. Their document order doesn’t matter – which loads first runs first So the user can read page content and get acquainted with the page immediately.īut there are also essential differences between them: Summaryīoth async and defer have one common thing: downloading of such scripts doesn’t block page rendering. Without script.async=false, scripts would execute in default, load-first order (the small.js probably first).Īgain, as with the defer, the order matters if we’d like to load a library and then another script that depends on it. LoadScript('/article/script-async-defer/small.js') LoadScript('/article/script-async-defer/long.js') long.js runs first because of async=false Let script = document.createElement('script') Whatever loads first (probably small.js) – runs first: Here’s an example similar to what we’ve seen with defer: two scripts long.js and small.js, but now with async instead of defer. A fully independent script that runs when loaded. The DOM and other scripts don’t wait for them, and they don’t wait for anything. In other words, async scripts load in the background and run when ready. …or after an async script (if an async script is short or was in HTTP-cache).

DOMContentLoaded may happen both before an async script (if an async script finishes loading after the page is complete).DOMContentLoaded and async scripts don’t wait for each other:.Other scripts don’t wait for async scripts, and async scripts don’t wait for them.The browser doesn’t block on async scripts (like defer).The async attribute means that a script is completely independent: But it has important differences in the behavior. The async attribute is somewhat like defer. The defer attribute is ignored if the tag has no src.

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