Trailing commas were common in language design long before JSON or even JavaScript existed as it simplifies machine generation of code while being comparatively

Trailing commas were common in language design long before JSON or even JavaScript existed as it simplifies machine generation of code while being comparatively trivial to handle in a parser, so a net win. The convenience it offers for diffing is just a manifestation of the positive interaction with grammars and language tools. The convention of humans using trailing commas in lists, along with one item per line, is relatively new, though. Stylistically, this used to be frowned upon as long definition lists made source files longer, slower to scroll through, and worsened code locality from the perspective of someone using, e.g., a 25 row terminal.