fay-text provides a Text whose values are represented as JavaScript strings for Fay, and as Data.Text for GHC. You can use this package instead of directly depending on text if you want to.
Fay has a special case that's applicable for fay-text. If a file has {-# LANGUAGE OverloadedStrings #-} GHC will replace all string literals with fromString lit. {-# LANGUAGE RebindableSyntax #-} tells GHC to use the fromString currently in scope instead of Data.String.fromString. With these two extensions enabled Fay outputs all string literals as JavaScript strings, which is the same representation as Fay.Text uses.
Note that you can mix modules using text literals and string literals, the behavior is local to the module.