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https://javascript.info/script-async-defer
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script.async = false;
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Dynamic scripts
We can also add a script dynamically using JavaScript:

 let script = document.createElement('script');
script.src = "/article/script-async-defer/long.js";
document.body.append(script); // (*)
The script starts loading as soon as it’s appended to the document (*).

Dynamic scripts behave as “async” by default.

That is:

They don’t wait for anything, nothing waits for them.
The script that loads first – runs first (“load-first” order).
 



let script = document.createElement('script');
script.src = "/article/script-async-defer/long.js";

script.async = false;

document.body.append(script);
For example, here we add two scripts. Without script.async=false they would execute in load-first order (the small.js probably first). But with that flag the order is “as in the document”:
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