Does Google Tag Manager execute document.write on custom HTML tags?

Question

Google's documentation clearly states that synchronously loading tags are not supported: https://support.google.com/tagmanager/answer/2787990?hl=en

However there is now a setting 'Support document.write' when creating a custom HTML tag. This implies that synchronously loading tags are now supported but I can't find any documentation to explain how the new setting works.

My question is: Does document.write actually get executed or does GTM do something clever like replace it with document.createElement in the background (as described here: http://www.stevesouders.com/blog/2012/04/10/dont-docwrite-scripts/)?

Answer 1

GTM actually overrides document.write for the scope of the executed tag (a custom HTML tag with support for document.write like you mentioned), as you can see on Chrome's devtools console:

document.write                                                     
function (){return m(g(arguments).join(""))}   gtm.js?id=GTM-someid:formatted:1455

Once GTM has finished the document.write call, it discards the override:

document.write
function write() { [native code] }

The implementation it provides uses "plain" insertion (e.g. document.createElement followed by appendChild or insertBefore) and a listener that runs when the script loads (e.g. onreadystatechange or onload) and executes the code that is placed after the document.write call in the tag definition.