As it is an array does it need  somewhere?!
I’m not sure how you call that JSON in JS, but be aware that this JSON is an array, which only contains a single object, i.e
property_value = json[property_name]
however, according to your output the code above is not the one creating the JSON (no hint of a wrapping array).
additional note, PHP associative arrays translate into JS plain objects, not JS arrays.