Doubt regarding javascript

#1

What does it mean of async and defer in this?

#2

Read about them here:
https://www.w3schools.com/tags/att_script_async.asp

https://www.w3schools.com/tags/att_script_defer.asp

#3

what happens when both async and defer is present

#4

It would be undefined behaviour, and should not be done.

#5
  • If async is present: The script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing)
  • If async is not present and defer is present: The script is executed when the page has finished parsing
  • If neither async or defer is present: The script is fetched and executed immediately, before the browser continues parsing the page
#6

You have just copied it from w3 schools site

#7

but i wants to know what happens when both async and defer is used beacuse it used in these lecture?

#8

The defer attribute may be specified even if the async attribute is specified, to cause legacy Web browsers that only support defer (and not async ) to fall back to the defer behavior instead of the synchronous blocking behavior that is the default.

Source: https://www.w3.org/TR/2011/WD-html5-20110525/scripting-1.html#attr-script-async

#9

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