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This article mainly introduces what is the difference between defer and async in js. It is very detailed and has a certain reference value. Friends who are interested must read it!

The difference between defer and async?

Both load external JS files asynchronously and do not block DOM parsing

Async is executed after the external JS is loaded and before the Load event is triggered when the browser is idle. Scripts marked async are not guaranteed to be executed in the order in which they are specified, and this attribute has no effect on inline scripts (that is, scripts without the * * "src" * * attribute).

Defer is executed after JS loading, after the entire document parsing is complete, and before triggering the DOMContentLoaded event. If the src attribute (that is, embedded script) is missing, it should not be used because it does not work in this case.

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