The zipcode-ja package provide simple useful very huge object which includes all postal codes in Japan.
This program makes the huge object split into many JSON files.
$ npm install zipcode-ja-s
$ mkdir tmpdest
$ ./node_modules/bin/split-zipcode-ja tmpdest
So, you will get many JSON files.
$ ls tmpdest
001.json 099.json 214.json 319.json 414.json 513.json 612.json 711.json
002.json 100.json 215.json 320.json 415.json 514.json 613.json 712.json
003.json 101.json 216.json 321.json 416.json 515.json 614.json 713.json
...
..
.