I'm currently using Ace Editor, but I couldn't find anything in the docs along the lines of retrieving the current selected line number and its text.
You can do this:
selectionRange = editor.getSelectionRange(); startLine = selectionRange.start.row; endLine = selectionRange.end.row; content = editor.session.getTextRange(selectionRange);
First, define "selected line". Selection in ace may be set across multiple lines. If you mean "no selection is set, current line is line where cursor blinks:"
var currline = editor.getSelectionRange().start.row; var wholelinetxt = editor.session.getLine(currline);
If you need exact selected text, see @parchment answer, I was about wrote the same, but it's not needed now.