Can't get Hello world example to work as a web-app - google-apps-script

Question

I am a total newbie to google-apps-script, but I can't believe the problem I'm having. I can't even get the Hello World! example to work as a web-app.
I have copied and pasted code from google's documentation website, and I still can't get it to work. (I tried more complicated stuff first, but then I started trying simpler and simpler code until I bottomed out with the below problem.)
When I publish the following code as a web-app:
function doGet() {
return ContentService.createTextOutput('Hello, world!');
}
Instead of seeing "Hello, world!" in the browser, I get the following error message:
The script completed but did not return anything.
I'm really at a loss here. I don't know how to try anything simpler, and again, the code snippet above was copied and pasted directly from the Content Service documentation page (https://developers.google.com/apps-script/reference/content/).
Any help with this problem is greatly appreciated.
Thanks in advance.

Answer 1

Thanks for the response Alan.
I created a new project and pasted the Hello world code into it, and it worked. I'll just work from this new project going forward.
...
Now that I have a few more minutes of experience, I think my problem was with version control. I thought that when I made a change in my code and updated the current version, then the current version published as a web app would reflect this change in my code. However, it seems that you have to create a new version of your code in order for your newly-published web-app to reflect the most recent changes in your code. My problem was that I was not creating new versions as I changed my code, so I kept accessing the old code which was always the same as when that version was first created.
(Using the Test web app for your latest code link (located in the Deploy as web app dialog which is accessed from the Publish menu) did reflect my most recent changes, but this was not adequate for my testing purposes since I was sending arguments to my web app url. (The latest code link only calls your web app with a read-only version of your web app url that has no arguments appended to it.) In order to properly test the web app with all arguments appended to the url, I needed to re-publish the new code under a different version number in order for those changes to be reflected in the web app that was published. Only then I could access this url that pointed to my latest code with all of my arguments appended.)
Again, just hitting the Update button in the Deploy as Web App dialog without specifying a new version does not actually update the current version with your new code. In order for your latest/newest code to be deployed you have to create a new version of your code that will reflect your latest changes. In order to create a new version, you first have to choose Manage versions... from the File menu and then save a new version that reflects your most recent code changes by hitting the Save New Version button. Once you have done this, you can choose to publish this new version in the Deploy as Web App dialog. The url for your web app will then access whichever version of your code that you last deployed/published.
I know that everyone on here probably already knows this, but I thought I'd clarify what my problem was (problem understanding the work-flow of the code-publish cycle) in case any newbie in the future also runs into this problem.
<<<<<UPDATE>>>>>
See the answer from Serge below about how to use the dev version of your web app url with parameters appended to it. In many cases, this can alleviate the need to create new (exec) versions of your web app as I described above if/when you are making only incremental changes to your code.
In order to get hold of your dev url in the browser address bar so you can copy it for further use (instead of getting the script.googleusercontent.com... url that is used to temporarily serve the content/output of your web app) just call an undefined function in your web app code. Then use the Test web app for your latest code link to call the dev version of your web app. This will cause an error page to be returned, and your dev url will then be available for copying in the browser address bar.

Answer 2

Following these steps I was able to get the Hello World Script to run:
Go to script.google.com
Paste your above code into the script area.
Save as Hello World.
Publish as version 1.0.
Go to the url it generates and "Hello, world!" is now displayed.
You can now also test the web app with the latest code when publishing.

Answer 3

Your statement that the .dev url is not able to support parameters is not exactly true (The latest code link only calls your web app with a read-only version of your web app url that has no arguments appended to it). Actually it does just like the exec one but you can only call this url directly from the browser (and not from another app) because only you can access the app using this special url.
demo code :
function doGet(e) {
var valToReturn = ContentService.createTextOutput('the parameter was '+e.parameter.val).setMimeType(ContentService.MimeType.TEXT);
return valToReturn;
}
The test .dev url with parameters goes like this :
https://script.google.com/macros/s/AKfycb___vWxs/dev?val='test'
and the return you get in your Browser is :