Regular Script Loading Inside of the head tag

• Browser fetches and executes the script first before parsing the HTML body.

• An error might occur here when you're manipulating the DOM because script is executed first and DOM doesn't exist yet.

Regular Script Loading at the end of the tag

• The Script will be fetched and executed when the HTML is parsed completely.

Async Script Loading

• Fetches the script asynchronously while the HTML is being parsed and immediately executes it when the script has been loaded completely.

Defer Script Loading

• The script is being fetched asynchronously and will only be executed when the HTML is parsed completely.