Online articles follow the same guidelines for printed articles. Include all information the online host makes available, including an issue number in parentheses.
Online scholarly journal articles without a DOI require the URL of the journal home page. Remember that one goal of citations is to provide your readers with enough information to find the article; providing the journal home page aids readers in this process.
Note: In August of 2011 the formatting recommendations for DOIs changed. DOIs are now rendered as an alpha-numeric string which acts as an active link. According to The APA Style Guide to Electronic References, 6th edition.
Tip: You should use the DOI format which the article appears with. So, if it is using the older numeric string, use that as the DOI. If, however, it is presented as the newer alpha-numeric string, use that as the DOI.
Because online materials can potentially change URLs, APA recommends providing a Digital Object Identifier (DOI), when it is available, as opposed to the URL. DOIs are an attempt to provide stable, long-lasting links for online articles. They are unique to their documents and consist of a long alphanumeric code. Many-but not all-publishers will provide an article's DOI on the first page of the document.
Some online bibliographies provide an article's DOI but may "hide" the code under a button which may read "Article" or may be an abbreviation of a vendor's name like "PubMed." This button will usually lead the user to the full article which will include the DOI. Find DOI's from print publications or ones that go to dead links with CrossRef.org's "DOI Resolver," which is displayed in a central location on their home page.
Thank you to the Online Writing Lab (OWL) at Purdue University.