Script placement differences between 1.x and 2.x

In 1.x a dynamically created inline script is added to the document head that:
1. Initializes a placeholder dataLayer array
2. Adds the external tag manager script to the page with the defer attribute. The browser will download the script, but not execute it until the DOM has finished parsing.

In 2.x a local script file is added to the bottom of the page that:
1. Initializes a placeholder dataLayer array.
2. Uses configuration passed with drupalSettings to add the external tag manager script to the page with the async attribute (since the script is at the bottom of the page, it is technically already defered). The browser will download the script and execute it when it is available (and the JS thread is not otherwise occupied).

The only functional difference is when the dataLayer variable is available to other scripts on the page. With 2.x, other libraries which use dataLayer will need to specify the google_tag library as a dependency to ensure they are correctly ordered to execute later when dataLayer is available. Any inline scripts inserted earlier in the page will not have access to the dataLayer variable. (You should get errors in the browser console if this were causing issues).
You can alter the google_tag library to place it in the head of the page so that dataLayer is available earlier in the page parsing.

adanbouzoua → changed the visibility of the branch 3451427-script-placement-differences to hidden.