ES6 中新加入了Map和WeakMap类用于存储 key - value pair。区别于原来的object, Map和WeakMap的key 可以是any value (objects or primitive values), 而 object 的 key 只能是 string 或者 symbol
Map
Create a map
let map = new Map([iterable]);- iterable
- An Array or other iterable object whose elements are key-value pairs (arrays with two elements, e.g. [[1, 'one'], [2, 'two']]). Each key-value pair is added to the new Map; null values are treated as undefined
Map v.s. Object
| Map | Object |
|---|---|
| key可以是 any value (object or primitive type) | key 只能是 String 或者 Symbol |
| Map 有 size property | Object 没有自带的 size property |
| Map 是一个 iterable, 所以它可以直接用 iterator 来迭代遍历 | Object不是iterable, 遍历一个Object需要用一个方法去遍历它的key, 再通过key得到value |
| Map的key不会跟Map自带的property发生collide | Object有prototype property, 可能添加的key会overwrite prototype里边的key。在ES5中可以用 map = Object.create(null) 来创建一个pure empty的object |
| 当add/delete operation比较频繁的时候，Map的performance比较好 |
Map的Properties和methods
| Properties | |
|---|---|
Map.prototype.size |
Returns the number of key/value pairs in the Map object. |
| Methods | |
Map.prototype.clear() |
Removes all key/value pairs from the Map object. |
Map.prototype.delete(key) |
Returns true if an element in the Map object existed and has been removed, or false if the element does not exist. Map.prototype.has(key) will return false afterwards. |
Map.prototype.entries() |
Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order. |
Map.prototype.forEach(callbackFn[, thisArg]) |
Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback. |
Map.prototype.get(key) |
Returns the value associated to the key, or undefined if there is none. |
Map.prototype.has(key) |
Returns a boolean asserting whether a value has been associated to the key in the Map object or not. |
Map.prototype.keys() |
Returns a new Iterator object that contains the keys for each element in the Map object in insertion order. |
Map.prototype.set(key, value) |
Sets the value for the key in the Map object. Returns the Map object. |
Map.prototype.values() |
Returns a new Iterator object that contains the values for each element in the Map object in insertion order. |
Map.prototype[@@iterator]() |
Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order. |
Useful Example
NaN as key
var myMap = new Map();
myMap.set(NaN, 'not a number');
myMap.get(NaN); // "not a number"
var otherNaN = Number('foo');
myMap.get(otherNaN); // "not a number"
Iterating Maps with for..of
var myMap = new Map();
myMap.set(0, 'zero');
myMap.set(1, 'one');
for (var [key, value] of myMap) {
console.log(key + ' = ' + value);
}
// 0 = zero
// 1 = one
Relation with Array objects
var kvArray = [['key1', 'value1'], ['key2', 'value2']];
// Use the regular Map constructor to transform a 2D key-value Array into a map
var myMap = new Map(kvArray);
myMap.get('key1'); // returns "value1"
// Use the Array.from function to transform a map into a 2D key-value Array
console.log(Array.from(myMap)); // Will show you exactly the same Array as kvArray
// Or use the keys or values iterators and convert them to an array
console.log(Array.from(myMap.keys())); // Will show ["key1", "key2"]
WeakMap
Create a WeakMap
let weakmap = new WeakMap([iterable]);- iterable
- An Array or other iterable object whose elements are key-value pairs (arrays with two elements, e.g. [[1, 'one'], [2, 'two']]). Each key-value pair is added to the new Map; null values are treated as undefined
WeakMap v.s. Map
| WeakMap | Map |
|---|---|
| WeakMap的key只能是object type | Map的key可以是任何type (object or primitive types) |
| WeakMap对key object的引用是弱引用，所以garbage collection机制可以将key object回收 | Map对key object的引用是strong reference |
| WeakMap keys are not enumerable | Map keys are enumerable. 所以如果要得到一个key 的list的话，应该考虑用Map |
WeapMap的properties和methods
| Properties | |
|---|---|
WeakMap.length |
The value of the length property is 0. |
WeakMap.prototype.delete(key) |
Removes any value associated to the key. WeakMap.prototype.has(key) will return false afterwards. |
WeakMap.prototype.get(key) |
Returns the value associated to the key, or undefined if there is none. |
WeakMap.prototype.has(key) |
Returns a Boolean asserting whether a value has been associated to the key in the WeakMap object or not. |
WeakMap.prototype.set(key, value) |
Sets the value for the key in the WeakMap object. Returns the WeakMap object. |
