When the browser loads HTML and comes across a <script>...</script> tag, it can’t continue building the DOM. It must execute the script right now. The same happens for external scripts <script src="..."></script>: the browser must wait until the script downloads, execute it, and only after process the rest of the page.

There are two <script> attributes that solve the problem for us: defer and async.

defer

The defer attribute tells the browser that it should go on working with the page, and load the script “in background”, then run the script when it loads.

async

The async attribute means that a script is completely independent:

• The page doesn’t wait for async scripts, the contents are processed and displayed.
• DOMContentLoaded and async scripts don’t wait for each other:
  • DOMContentLoaded may happen both before an async script (if an async script finishes loading after the page is complete)
  • …or after an async script (if an async script is short or was in HTTP-cache)
• Other scripts don’t wait for async scripts, and async scripts don’t wait for them.

So, if we have several async scripts, they may execute in any order. Whatever loads first – runs first

Hope it can help you :)

Cherry pick from concept