If the syntax allows for a trailing separator, I'd go with opening delimiters and separators on the right, with a trailing separator, and the closing delimiter

If the syntax allows for a trailing separator, I'd go with opening delimiters and separators on the right, with a trailing separator, and the closing delimiter on its own line. The reasoning behind that is that it produced the cleanest diffs with source control - especially if you consider changing the first/last/only entry in a collection. Consider the diff from removing the first phone number from your collection above, and compare that to removing it from "phoneNumbers": [ "212 555-1234", "646 555-4567", ],