const map = new Map();
map.set("foo", "bar");
console.log(map.get("foo")); //logs "bar"
const animalSounds = new Map();

animalSounds.set("dog", "woof");
animalSounds.set("cat", "meow");
animalSounds.set("frog", "ribbit");

console.log(animalSounds.size); //logs 3
console.log(animalSounds.has("dog")); //logs true

animalSounds.delete("dog");

console.log(animalSounds.size); //logs 2
console.log(animalSounds.has("dog")); //logs false

animalSounds.clear();
console.log(animalSounds.size); //logs 0

usersMap = new Map();
usersMap.set(1, "sally");
usersMap.set(2, "bob");
usersMap.set(3, "jane");

console.log(usersMap.get(1)); //logs "sally"
usersMap.forEach(function(username, userId) {
  console.log(userId, typeof userId); //logs 1..3, "number"
  if (userId === 1) {
    console.log("We found sally.");
  }
});

//如果用for...of方式遍历，每次返回的是一个Array
for (data of usersMap) {
  console.log(data); //Array [1,"sally"]
}

const obj, map;
map = new Map();
obj = { foo: 'bar' };
map.set(obj, 'foobar');
obj.newProp = 'stuff';
console.log(map.has(obj)); //logs true
console.log(map.get(obj)); //logs "foobar"