現在已知有 object 儲存 key 的資料，array 儲存 index 的資料，但這對現實世界來說還不夠。
Map
map 跟 object 一樣，唯一不同的是 key 可以是任何類型的值。map 的方法如下：
new Map() – creates the map.
map.set(key, value) – stores the value by the key.
map.get(key) – returns the value by the key, undefined if key doesn’t exist in map.
map.has(key) – returns true if the key exists, false otherwise.
map.delete(key) – removes the value by the key.
map.clear() – clears the map
map.size – returns the current element count.
let map = new Map();
map.set('1', 'str1'); // a string key
map.set(1, 'num1'); // a numeric key
map.set(true, 'bool1'); // a boolean key
// remember the regular Object? it would convert keys to string
// Map keeps the type, so these two are different:
alert( map.get(1) ); // 'num1'
alert( map.get('1') ); // 'str1'
alert( map.size ); // 3
// object key
let john = { name: "John" };
// for every user, let's store their visits count
let visitsCountMap = new Map();
// john is the key for the map
visitsCountMap.set(john, 123);
alert( visitsCountMap.get(john) ); // 123
// object with object key
let john = { name: "John" };
let visitsCountObj = {}; // try to use an object
visitsCountObj[john] = 123; // try to use john object as the key
// That's what got written!
alert( visitsCountObj["[object Object]"] ); // 123
// before map exist
// we add the id field
let john = { name: "John", id: 1 };
let visitsCounts = {};
// now store the value by id
visitsCounts[john.id] = 123;
alert( visitsCounts[john.id] ); // 123
// chain
map.set('1', 'str1')
.set(1, 'num1')
.set(true, 'bool1');
Map from Object
// array of [key, value] pairs
let map = new Map([
['1', 'str1'],
[1, 'num1'],
[true, 'bool1']
]);
// Object.entries() returns [ ["name","John"], ["age", 30] ]
let map = new Map(Object.entries({
name: "John",
age: 30
}));
Iteration over Map
map.keys() – returns an iterable for keys,
map.values() – returns an iterable for values,
map.entries() – returns an iterable for entries [key, value], it’s used by default in for..of.
let recipeMap = new Map([
['cucumber', 500],
['tomatoes', 350],
['onion', 50]
]);
// iterate over keys (vegetables)
for (let vegetable of recipeMap.keys()) {
alert(vegetable); // cucumber, tomatoes, onion
}
// iterate over values (amounts)
for (let amount of recipeMap.values()) {
alert(amount); // 500, 350, 50
}
// iterate over [key, value] entries
for (let entry of recipeMap) { // the same as of recipeMap.entries()
alert(entry); // cucumber,500 (and so on)
}
// map 有內建 forEach 方法
// runs the function for each (key, value) pair
recipeMap.forEach( (value, key, map) => {
alert(`${key}: ${value}`); // cucumber: 500 etc
});
Set
set 是一個值的集合，裡面的值不能重複。替代方案是插入值時用 arr.find() 檢查是否有該值，但會造成性能變差。
new Set(iterable) – creates the set, and if an iterable object is provided (usually an array), copies values from it into the set.
set.add(value) – adds a value, returns the set itself.
set.delete(value) – removes the value, returns true if value existed at the moment of the call, otherwise false.
set.has(value) – returns true if the value exists in the set, otherwise false.
set.clear() – removes everything from the set.
set.size – is the elements count.
let set = new Set();
let john = { name: "John" };
let pete = { name: "Pete" };
let mary = { name: "Mary" };
// visits, some users come multiple times
set.add(john);
set.add(pete);
set.add(mary);
set.add(john);
set.add(mary);
// set keeps only unique values
alert( set.size ); // 3
for (let user of set) {
alert(user.name); // John (then Pete and Mary)
}
Iteration over Set
set.keys() – returns an iterable object for values,
set.values() – same as set.keys, for compatibility with Map,
set.entries() – returns an iterable object for entries [value, value], exists for compatibility with Map.
let set = new Set(["oranges", "apples", "bananas"]);
for (let value of set) alert(value);
// the same with forEach:
set.forEach((value, valueAgain, set) => {
alert(value);
});
// delete object
let john = { name: "John" };
// the object can be accessed, john is the reference to it
// overwrite the reference
john = null;
// the object will be removed from memory
// store in array object still exist
let john = { name: "John" };
let array = [ john ];
john = null; // overwrite the reference
// john is stored inside the array, so it won't be garbage-collected
// we can get it as array[0]
// store in map object still exict
let john = { name: "John" };
let map = new Map();
map.set(john, "...");
john = null; // overwrite the reference
// john is stored inside the map,
// we can get it by using map.keys()
// weakMap / weakSet 只能使用 object 當 key 值
let weakMap = new WeakMap();
let obj = {};
weakMap.set(obj, "ok"); // works fine (object key)
// can't use a string as the key
weakMap.set("test", "Whoops"); // Error, because "test" is not an object
// store in weakMap object desroty
let john = { name: "John" };
let weakMap = new WeakMap();
weakMap.set(john, "...");
john = null; // overwrite the reference
// john is removed from memory!
// 如果物件被移除，儲存值也會被移除
weakMap.set(john, "secret documents");
// if john dies, secret documents will be destroyed automatically
// 手動清理不要的物件
let john = { name: "John" };
// map: user => visits count
let visitsCountMap = new Map();
// john is the key for the map
visitsCountMap.set(john, 123);
// now john leaves us, we don't need him anymore
john = null;
// but it's still in the map, we need to clean it!
alert( visitsCountMap.size ); // 1
// and john is also in the memory, because Map uses it as the key
// weakMap 當物件消失值也會消失
let john = { name: "John" };
let visitsCountMap = new WeakMap();
visitsCountMap.set(john, 123);
// now john leaves us, we don't need him anymore
john = null;
// there are no references except WeakMap,
// so the object is removed both from the memory and from visitsCountMap automatically
// weakSet exaample
let messages = [
{text: "Hello", from: "John"},
{text: "How goes?", from: "John"},
{text: "See you soon", from: "Alice"}
];
// fill it with array elements (3 items)
let unreadSet = new WeakSet(messages);
// use unreadSet to see whether a message is unread
alert(unreadSet.has(messages[1])); // true
// remove it from the set after reading
unreadSet.delete(messages[1]); // true
// and when we shift our messages history, the set is cleaned up automatically
messages.shift();
// no need to clean unreadSet, it now has 2 items
// (though technically we don't know for sure when the JS engine clears it)
weakMap 只有以下方法：
weakMap.get(key)
weakMap.set(key, value)
weakMap.delete(key)
weakMap.has(key)
weakSet
It is analogous to Set, but we may only add objects to WeakSet (not primitives).
An object exists in the set while it is reachable from somewhere else.
Like Set, it supports add, has and delete, but not size, keys() and no iterations.