const jsonToMap = (json) => new Map(Object.entries(JSON.parse(json)));
const json = '{"user1":"John","user2":"Kate","user3":"Peter"}';
const map = jsonToMap(json);
// Kate
console.log(map.get('user2'));
// Map(3) { 'user1' => 'John', 'user2' => 'Kate', 'user3' => 'Peter' }
console.log(map);