getPrefecturesFromRegionId.js

var data = [
    {
        regionId: 1,
        regionName: "Kanto",
        prefectures: [
            {id: 1, name: "Tokyo", population: 10000},
            {id: 2, name: "Kanagawa", population: 20000},
            {id: 3, name: "Chiba", population: 300},
            {id: 4, name: "Gunma", population: 20}
        ]
    },
    {
        regionId: 2,
        regionName: 'Tohoku',
        prefectures: [
            {id: 1, name: "Aomori", population: 1500},
            {id: 2, name: "Iwate", population: 200},
        ]
    }
]

function getPrefecturesFromRegionId(regionId){
    return data.reduce(function(p,c,idx,arr){
        if (c.regionId === regionId){
            return p.concat(c.prefectures)
        }
        return p
    },[]).map(function(e){
        return {id: e.id, name: e.name}
    })
}
console.log(getPrefecturesFromRegionId(1))

//[ { id: 1, name: 'Tokyo' },
//  { id: 2, name: 'Kanagawa' },
//  { id: 3, name: 'Chiba' },
//  { id: 4, name: 'Gunma' } ]