The only things JSON has over xml is that it’s easier to write a parser for it and the format is less verbose and less complicated. There are extensions to JSON that can add features that xml has and the JSON spec doesn’t have. Overall the xml spec is bigger and has more features but that also makes it overkill for many of the cases that it would be used in.
It was originally meant as a better JavaScript and it was. It failed when none of the other browsers expressed interest in supporting it. It languished for a while and then was taken up by the Flutter team. At the time Flutter took it up it was somewhere around the level of Java 8 in features but not quite on par. Since then it’s seen some massive improvements to the type system and language. It’s completely null sound, not just null safe like Kotlin. It recently got records/tuples and one of the more capable pattern matching syntaxes I’ve ever seen in a functional imperative hybrid language. The next stable version of dart will introduce a compiler macro system that is very promising. The syntax isn’t always the prettiest due to it trying to not totally break old code. I do think that it offers a wide range of modern language features that competes heavily with Swift and Kotlin in the mobile space.