It makes clever use of pluralization and natural language patterns so the output strings read like a natural language utterance instead of a computer-generated string, for example it generates these outputs for 0, 1 and 1,000 items, respectively:
“Annie took no photos on December 14, 2014.”
“Annie took one photo on December 14, 2014.”
“Annie took 1,000 photos on December 14, 2014.”
FormatJS applies similar logic to date representations. Today (21-12-2014) the abstract date “2014-11-21” for instance is represented as “last month”, “1 month ago” or “31 days ago”.