Today, nearly all journal articles are read online. The way APA format is used today, if something exists or will soon exist in paper, even if you have read the online or electronic version, you should reference it just like a paper source.
This also extends to most online-only journals, especially if you give the DOI number (which enables an immediate link to the article’s page). You can find information about referencing online-only journals on the Journal articles
page in this guide.