I've use Google Apps Script before, but haven't bene programming for oabout a year now.
For a new project I would like to know if I should continue using Google Apps script or switch to HTMLService.
So first question is if Google Apps Script still is and well be supported by Google.
Second question is what are the main differences between HTMLService and Google Apps Script that make it worth switching.
If you mean should you use HTMLService or UIService within Google Apps Script, it would probably be best to use HTMLService as UIService is deprecated.
UIService will appear in auto-complete until 30th June, but not after.
http://googleappsdeveloper.blogspot.co.uk/2014/12/speeding-up-htmlservice.html
For first Question: Yes, The Google app script used to make google products as custom like google spread sheet , google docs and we can insert the script into google sites so that we can share output through website.
For second Question: In Google app script the ClassUIApp is deprecated in that place we can using HTML services https://developers.google.com/apps-script/reference/ui/ui-app
In the beginning Google App Script, all the code was written in one file one with .gs extension which included the code for User Interface, styling and business logic which made it complex as the application moved from small scale to enterprise level.
Also there was a restriction that you have to use the UI components from App Script (UiApp) which was not welcomed by the existing community of web developers as it was limited and needed to be learned and could not offer all the features a programmer may require.
Therefore in order to make it popular among existing community of developers App Script gave the ability of writing HTML pages as part of the project which was very helpful as it is widely used for designing all kinds of UI.
So, as you can see here that Google has made a shift, and hence HTMLservice is the future.
Google Apps Script was design for interact with google apps, so, if you are working with google drive, calendar, maps. You must use apps script, It's is easy and fast to implement.
But if you don't need use Google Apps may be is better use another software because how I told you before, Apps Script was created for interact with Google Apps and works awesome.
Google say: "The HTML service lets you serve web pages that can interact with server-side Apps Script functions. It is particularly useful for building web apps or adding custom user interfaces in Google Docs, Sheets, and Forms. You can even use it to generate the body of an email." https://developers.google.com/apps-script/guides/html/
The purpose is different, what do you want to do?
Been following updates from Google in regard to GAS for over 6 months now. Google is putting a lot of resource into GAS and I don't think there should be any concern about it being deprecated.
And HTMLService is a class that can be called from inside the script editor so I don't know why one would need to choose one or the other.
Just go into the script editor and press control + space bar at the same time and you'll see all the classes available to you in alphabetical order. Scroll down to H and you'll see HTMLService. I haven't used it yet but it's there to work with.
