オブジェクトのループ

キーと値の両方

const obj = { name: 'john', age: 10, address: 'us' };
for (const [key, val] of Object.entries(obj)) {
  console.log(key, val);
}

キーだけ

const obj = { name: 'john', age: 10, address: 'us' };
for (const key of Object.keys(obj)) {
  console.log(key);
}

値だけ

const obj = { name: 'john', age: 10, address: 'us' };
for (const val of Object.values(obj)) {
  console.log(val);
}

空オブジェクトの判定

const obj = {};
console.log(Object.entries(obj).length === 0);
console.log(Object.keys(obj).length === 0);
console.log(Object.values(obj).length === 0);

変換

const users = { Alice: 3, Bob: 1, Carol: 2 };
const users2 = Object.entries(users)
 .reduce((obj, [key, val]) => {
   obj[key] = val * 2;
   return obj;
 }, {});
// users2 = { Alice: 6, Bob: 2, Carol: 4 }