As we all know the Siemens debugger that we use to write the java script in lacks lots of features. There is no auto complete. No even a really good help that indicates if you spell something wrong. E.g. using big and small letters in the wrong way. If so when starting your code it just terminates without any reason. Enojng.... There are some helpful things that you can use to simplify the coding and debugging:

1) To test java scripts in general, this site is very good. You have to be registered but to write small snippets for test is free. I use this one: https://playcode.io/, I highly recommend it. Here you can also see how a modern development platform works. 
playcode

2) From TIA19 it should be possible to use Visual studio for writing code to link it to the TIA portal. I did up to now not test this but that can also be a solution. You can read more abut this in this good post:  Link to post covering this

3) Use the Chrome debug tool. This is Veeery helpful: This was what I made me to get it to work:

* Start SIMATIC runtime manager as administrator. Press the settings on the right hand side in the tool bar. Under script Debugger select set enable. Port should be default 9222. then press ok to save.
* If you have a running application in the runtime manager stop it and restart it again.
* Start simulating your application from TIA.
* Access the webserver to run your application in Chrome. I use https://'computerName'/WebRh
* Navigate to the page where you want to debug.
* Press + to add a new window in your web browser: Write chrome://inspect/
* In the chrome debugger: Press configure and add this target: 127.0.0.1:9222
* Wait until the debugger detects remote target 127.0.0.1. Can take some time.
* There will be two target traces showing up. For the "DefaultUser@localhost VCS_3 Events ( DefaultUser@localhost VCS_3 Events". It is normally the second listed litem. press inspect for this item. This will show the Devtools.

* Navigate to the Source tab. here you can now see your source code. If you don't see it close the window and redo opening the devtools again as described above. Sometimes you need to do this several times.
* Under source you will find all your code blocks listed. E.g. if you did write code in a global module it will show up as: /global_modules/"module name". where modulename is the name you have set for your global module. Press the global module that you want to debug. The code window will then show your code.
* You can now set a break point on one line. by clicking on the line number. Line number will then be marked in blow arrow shape.
* Go back to the HMI runtime in the other Chrome window. Trig the function where you set the breakpoint. E.g. hitting a button or whatever event you used for trigging.

* Program will stop execution at the line you set the break point. The line will be marked yellow. You can now hover over tags to see the actual values. You can add variables or objects to the Watch list to see all the data.
* a tip is also to activate the Pause on uncaught error and Pause on caught error. Then the execution will stop on the line causing the error. (very helpful)
* There are tool buttons on the left hand side where you can select to step to next line in code or continue the execution after stopping.

* You can also look in the console by selecting this menu item. Next to the source item.
Anywhere In your code you can then use consol.log to log any data. E.g:

  consol.log("Hello world");
  consol.log(MyArray);
  consol.log(MyObject)
  consol.log("Värdet på min variabel är: "+ MyInt.
  consol.log(`Another way to format a string: ${MyInt}`

All this messages will be shown under the console. The good thing is that you can log whole arrays and whole objects.

You will get into it fast. The Debug tool is very self explaining.

One more thing. Do not make any change in the code in the code window if the debug tool. make changes in TIA and then restart the simulation again. After restart the simulation you must also start the debug tools as it looses connection when the HMI simulation is downloaded.

Good luck with your coding. Just let me know if you have any questions.

//Mattias Lindh