Do you really know the difference between Set, Map, WeakSet and WeakMap?

Set (Set)

Is a data structure called collections (new in ES6)

Array.from, or ES6… Extended operators)

const items = new Set([1.2.3.1])
const array = Array.from(items)
console.log(array)	/ / [1, 2, 3]
/ / or
const arr = [...items]
console.log(arr)	/ / [1, 2, 3]
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Is itself a constructor used to generate a Set data structure
```
new Set([iterable])
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```

Members are unique, unordered, and non-repetitive

const s = new Set([1.2.3.5.4.3.2.1]) //1 2 3 5 4
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[value, value], the key value is the same as the key name (or only the key value, no key name)

Allows you to store a unique value of any type, either a primitive value or an object reference

new Set({a:"123".b:"1234"}){a:"123",b:"1234"} This is not iterable
const s = new Set(a); s.add({a:"123".b:"1234"})// It works
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You can iterate with for of

const s = new Set([1.2.3.5.4.3.2.1]) //1 2 3 5 4
for (let i of s) {
    console.log(i)	// 1 2 3 5 4
}
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When a value is added to a Set, no type conversion occurs

let s = new Set()
s.add(1)
s.add('1')
console.log([...s])	/ / / 1, "1"
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The Set instance attributes are:

Constructor: a constructor

Size: number of elements

let set = new Set([1.2.3.5.4.3.2.1])
console.log(set.size)	/ / 3
console.log(set.length)	// undefined
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Note: There is no length attribute

You can add, delete, has, and clear
1. Add (value) : adds a value, which is equivalent to push in an array
2. Delete (value) : deletes the value in the collection
3. Has (value) : Determines whether a value exists in the set
4. Clear () : Clears the collection
```
let set = new Set()
set.add(1).add(2).add(3)
set.has(1)	// true
set.has(4)	// false
set.delete(1)	
set.has(1)	// false
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```
Keys values entries forEach (Traversal order: insert order)
1. Keys () : Returns an iterator containing all the keys in the collection
2. Values () : Returns an iterator containing all the worth iterators in the setKeys with values () ()It’s the same thing becauseKey value and key nameIs consistent) as shown in the picture:
3. Entries () : Returns a key-value iterator containing all the elements of the Set object
```
The key and value structure is shown in the screenshot belowCopy the code
```
4. ForEach (callbackFn, thisArg) : used to perform callbackFn on a collection member. If thisArg is provided, this in the callback would be this.No return value
Set can be traversed by default, and the default iterator generator is the values() method
```
Set.prototype[Symbol.iterator] === Set.prototype.values	// true
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```
Set can use map and filter methods

Intersect and Union can be easily implemented with Set.

let set1 = new Set([1.2.3.5])
let set2 = new Set([4.3.2.1])
let intersect = new Set([...set1].filter(value= > set2.has(value)))
let union = new Set([...set1, ...set2])
console.log(intersect)	// Set(3) {1, 2, 3}
console.log(union)		// Set(5) {1, 2, 3, 5, 4}
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WeakSet

Members are objects
Members are weak references that can be collected by the garbage collection mechanism and can be used to hold DOM nodes without causing memory leaks
Can’t traverse
The differences with Set are as follows:
- WeakSet can only store object references, not values, but both Set values and objects can be stored
- The object values stored in the WeakSet object are weakly referenced, that is, the garbage collection mechanism does not consider the application of WeakSet to the object. If there is no other variable or attribute referencing the object value, the object will be garbage collected (not considering the object still exists in the WeakSet), so, The number of member elements in the WeakSet object depends on whether the garbage collection mechanism is running. The number of members before and after the operation may be inconsistent. After the end of traversal, some members may not be taken (recycled by the garbage), and the WeakSet object cannot be traversed (ES6 specifies that WeakSet cannot be traversed). There’s no way to get all the elements it contains
WeakSet instance attributes are:
- Any object that has an Iterable interface can be used as an argument
The operation methods include :add, delete, has,Clear method does not have
1. Add (value) : Add an element value to the WeakSet object
2. Has (value) : Determines whether a Value is included in a WeakSet object
3. Delete (value) : deletes the value of the element
4. Clear () : Clears all elements, noting that this method is deprecated
```
var wst = new WeakSet(a)var obj = {}
var abc = {}
wst.add(window)
wst.add(obj)
wst.has(window)	// true
wst.has(abc)	// false
wst.delete(window)	// true
wst.has(window)	// false
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```

The Map (dictionary)

It’s a data structure called a dictionary
It’s essentially a set of key-value pairs, sort of a set
You can iterate, you can do a lot of things and you can convert to a lot of different data formats
The Map instance attributes are:
1. Constructor: a constructor
2. Size: Returns the number of elements contained in the dictionary
```
const map = new Map([['name'.'long'],
  ['age'.12]]); map.size/ / 2
map.length /// undefined
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```
- Note: There is no length attribute
The Map operation method has: set, get, has, delete, clear
1. Set (key, value) : Adds a new element to the dictionary
2. Get (key) : Finds a specific value by key and returns it
3. Has (key) : Checks whether the key exists in the dictionary
4. Delete (key) : removes the corresponding data from the dictionary by key
5. Clear () : Removes all elements from this dictionary
```
const m = new Map(a)const o = {name: 'long'}
m.set(o, 'liangliang')
m.get(o)	// liangliang
m.has(o)	// true
m.delete(o)	// true
m.has(o)	// false
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```

Traversal methods are: the Keys and values, entries, forEach

Keys() : Returns all key names contained in the dictionary as iterators
Values () : Returns all values contained in the dictionary as iterators
Entries () : Returns an iterator for all members
ForEach (callbackFn,thisArg) : iterates through all members of the dictionary, performing callbackFn on the collection member. If thisArg is provided,this in the callback will be the thisArg argument, with no return value

const map = new Map([['name'.'long'],
        ['age'.16]]);console.log(map.entries())//MapIterator {"name" => "long", "age" => 16}
console.log(map.keys())//MapIterator {"name", "age"}
console.log(qmap.values())//MapIterator {"long", 16}

const reporter = {
  report: function(key, value) {
    console.log("Key: %s, Value: %s", key, value); }};let map = new Map([['name'.'long'],
    ['age'.16]])// Note that the this of the forEach callback refers to reporter
map.forEach(function(value, key, map) {
  this.report(key, value);
}, reporter);
// Key: name, Value: long
// Key: age, Value: 16
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The default Map traverser interface (symbol. iterator property) is the entries method

const map = new Map()
map[Symbol.iterator] === map.entries // true
Map.prototype[Symbol.iterator]  === Map.prototype.entries
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Differences between sets and dictionaries:
- Common: Collections, dictionaries can store values that are not duplicated
- Differences: Collections store elements as [value, value], dictionaries as [key, value]
Matters needing attention
- The Map structure only treats references to the same object as the same key, which is easy to mistake
```
const map = new Map(a); map.set(['a'].555);
map.get(['a']) // undefined
var scs = ['a'];
map.set(scs , Awesome!);
map.get(scs)/ / 666
// Set and get methods, ostensibly for the same key, are actually two values,
// The memory address is different, so the get method cannot read the key, returns undefined,
// The second example is fine
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```
- If the Map’s key is a value of a simple type (number, string, Boolean), the Map will treat the two values as one key as long as they are strictly equal, such as 0 and -0 being one key, and true and the string true being two different keys. Also,undefined and NULL are two different keys. Although NaN is not strictly equal to itself, Map treats it as the same key.

Map and other data structures are converted as follows:

Turn the Map Array

const map = new Map([[1.1], [2.2], [3.3]])
console.log([...map])	// [[1, 1], [2, 2], [3, 3]]
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Array to the Map

const map = new Map([[1.1], [2.2], [3.3]])
console.log(map)	// Map {1 => 1, 2 => 2, 3 => 3}
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Turn the Map Object

Since the key names of Object are strings and the key names of Map are objects, non-string keys are converted to string keys during conversion.

function mapToObj(map) {
    let obj = Object.create(null)
    for (let [key, value] of map) {
        obj[key] = value
    }
    return obj
}
const map = new Map().set('name'.'long').set('age'.16)
mapToObj(map) // {name: "long", age: "16"}
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Turn the Object Map

function objToMap(obj) {
    let map = new Map(a)for (let key of Object.keys(obj)) {
        map.set(key, obj[key])
    }
    return map
}
objToMap({'name': 'long'.'age': 16})
// Map {"name" => "long", "age" => 16}
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The Map to JSON

function mapToJson(map) {
    return JSON.stringify([...map])
}
let map = new Map().set('name'.'LONG').set('age'.16)
mapToJson(map)	// [["name","LONG"],["age",16]]
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JSON to turn the Map

function objToMap(obj) {
    let map = new Map(a)for (let key of Object.keys(obj)) {
        map.set(key, obj[key])
    }
    return map
}
function jsonToStrMap(jsonStr) {
  return objToMap(JSON.parse(jsonStr));
}
jsonToStrMap('{"name": "long", "age": 16}') 
// Map {"name" => "long", "age" => 16}
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WeakMap

Only objects (except null) are accepted as key names, and values of other types are not accepted as key names.
The key name is a weak reference, the key value can be arbitrary, the object that the key name refers to can be garbage collected, in which case the key name is invalid.
The methods are get, set, has, and delete.
Note that WeakMap weakly references only the key name, not the key value. Key values are still normal references.
In WeakMap, the reference of each key to its referenced object is weak. If there is no other reference and the key references the same object, the object will be garbage collected (the corresponding key becomes invalid). Therefore, the key of WeakMap is not enumerable.
Properties:
- Constructor: a constructor

Methods:

Has (key) : checks whether there is an object associated with the key
Get (key) : returns the object associated with the key (undefined)
Set (key) : Sets a group of key associated objects
Delete (key) : deletes the object associated with the key

let myElement = document.getElementById('logo');
let myWeakmap = new WeakMap(a); myWeakmap.set(myElement, {timesClicked: 0});
myElement.addEventListener('click'.function() {
  let logoData = myWeakmap.get(myElement);
  logoData.timesClicked++;
}, false);
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conclusion

These concepts are newly added, and it is difficult to remember them all at once. Our best memory is to use them, and we can use them successfully in our daily work
Finally, if you like it, you can give it a thumbs up. Thank you

Do you really know the difference between Set, Map, WeakSet and WeakMap?

Set (Set)

WeakSet

The Map (dictionary)

WeakMap

conclusion

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