`async` & `defer` Attributes

Usually javascript files interrupt the parsing of HTML document. To prevent this 2 special attributes of the <script> elements are used.


<script async src="script.js">
  • The async attribute is used to indicate to the browser that the script file can be executed asynchronously.

  • Therefore the HTML parser does not need to pause and wait for the JS code to load, it is intead fetched in parallel.

  • It is only available for externally located script files.


<script defer src="script.js">
  • The defer attribute tells the browser to only execute the script file once the HTML document has been fully parsed.

  • The js file can be downloaded but it does not executes until the whole HTML is parsed.

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