Weakmap and Map

1// Object ========================
2// Remember all keys are strings or will be converted to strings
3
4// Array ========================
5
6// WeakMap ========================
7
8// Map ========================
9let john = { name: "John" };
10let map = new Map();
11map.set('1', 'str1');   // a string key
12map.set(1, 'num1');     // a numeric key
13map.set(true, 'bool1'); // a boolean key
14map.set(john, 123); // an object key *Using objects as keys is one of most notable and important Map features.
15
16// remember the regular Object? it would convert keys to string
17// Map keeps the type, so these two are different:
18alert( map.get(1)   ); // 'num1'
19alert( map.get('1') ); // 'str1'
20alert( map.size ); // 3
21map.keys() – returns an iterable for keys,
22map.values() – returns an iterable for values,
23map.entries() – returns an iterable for entries [key, value], it’s used by default in for..of.
24// iterate over keys (vegetables)
25for (let vegetable of recipeMap.keys()) {
26  alert(vegetable); // cucumber, tomatoes, onion
27}
28
29// iterate over values (amounts)
30for (let amount of recipeMap.values()) {
31  alert(amount); // 500, 350, 50
32}
33
34// iterate over [key, value] entries
35for (let entry of recipeMap) { // the same as of recipeMap.entries()
36  alert(entry); // cucumber,500 (and so on)
37}
38
39// runs the function for each (key, value) pair
40recipeMap.forEach( (value, key, map) => {
41  alert(`${key}: ${value}`); // cucumber: 500 etc
42});
43
44// Instantiating in bulk
45let map = new Map([
46  ['1',  'str1'],
47  [1,    'num1'],
48  [true, 'bool1']
49]);
50
51// Create a map from object
52let obj = {
53  name: "John",
54  age: 30
55};
56let map = new Map(Object.entries(obj));
57
58// Create from map
59let prices = Object.fromEntries([
60  ['banana', 1],
61  ['orange', 2],
62  ['meat', 4]
63]);
64// now prices = { banana: 1, orange: 2, meat: 4 }
65alert(prices.orange); // 2
66
67// Map to Object
68let map = new Map();
69map.set('banana', 1);
70map.set('orange', 2);
71map.set('meat', 4);
72let obj = Object.fromEntries(map.entries()); // make a plain object (*)
73// done!
74// obj = { banana: 1, orange: 2, meat: 4 }
75alert(obj.orange); // 2
76
77// Caveats - chaining: every map.set call returns the map itself, so we can “chain” the calls:
78map.set('1', 'str1')
79  .set(1, 'num1')
80  .set(true, 'bool1');
81
82
83