How to access full email source code in thunderbird message_display_action extension?

I am building an extension in Thunderbird with UI element message_display_action for my school project. I am wondering if I can access full email source code from java script file that I am using for building up my html page when pressing on button my extension. I found a funciton named getFull(messageId) in documentation at https://webextension-api.thunderbird.net/ but I don't understand how to use that function and I don't even know what messageId is. I know it is a integer but I don't know how to get that integer for a specific email. I entered permission (messagesRead) in my manifest file of extension for reading emails but i still don't know how to use that function. I didn't find any examples or tutorials on the internet so if anyone can help me or atleast point me to the right direction.

Getting the id of the currently displayed message

You should take a look at https://github.com/thundernest/sample-extensions/tree/master/messageDisplay, which is an example how to get the id of the currently displayed message from the messageDisplayAction using messageDisplay.getDisplayedMessage().

See also https://webextension-api.thunderbird.net/en/latest/messageDisplay.html.

messages.getFull()

Didn't find a small example using that API. As a hint you should note that this function works on the different MIME parts a mail consists of.

You will not get the complete source of the email that way. To get the raw source you would need to use messages.getRaw(). But you should probably not use getRaw() unless you really need it, because you would need to handle the complete parsing of the message yourself.