App Proxies And Their Sub Paths

Shopify Partner
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The documentation on app proxies states:

Merchants can change the sub path or sub path prefix of app proxies. Merchants change the sub path prefix by toggling between a set of values provided by Shopify (/apps, /a, /community, or /tools).

When that happens, is there any way the Shopify app can know what the sub path was changed to?  I'm going to have some Javascript code that will be hitting a proxied URL, and I need to always know where it is.