Well I'll finally try to answer your question, hoping you'll find it clear enough... ;-)
First thing first :
var ss = SpreadsheetApp.open()
does indeed appear in the autocomplete after the dot, what you didn't pay attention to is that the argument is a file, ie an object returned by an appropriate statement. Now let's see how to get it and , with this example how the other parameters of this file can be obtained :
function myFunction() {
var files = DocsList.find('thisisthesheetIwant');// this is an array of file objects that include the term 'thisisthesheetIwant'
var file = files[0];// I take the first one
var filename = file.getName();//and get its name
var fileId = file.getId();// its ID
var fileurl = file.getUrl();//and its url
// then I show the results in the logger
Logger.log('number of file found = '+files.length+'\n\n'+filename+'\n\n'+fileId+'\n\n'+fileurl+'\n\n')
var ss = SpreadsheetApp.open(file);// using that file object I can open a spreadsheet
var content = ss.getSheets()[0].getDataRange().getValues().toString();// and get the whole content of the first sheet
Logger.log('content = '+content);// there it is
}
The spreadsheet with its code is available here so you can test it by yourself, I named it thisisthesheetIwant
hoping you don't have any file with a similar name or content since it wouldn't work as I expected if more than 1 file was returned.
Look at the logger and I hope it will answer your question(s). it should appear like this below :
And the sheet itself is like this :
EDIT : note that the ID and the URL have a common part, the url is what you can see in your Browser's adress bar, the ID is just a part of it. Now you can open the same spreadsheet with
SpreadsheetApp.openById(fileId)