map.js

alert = console.log

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

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

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)
}

// runs the function for each (key, value) pair
recipeMap.forEach( (value, key, map) => {
  alert(`${key}: ${value}`); // cucumber: 500 etc
});

{
	let obj = {
    name: "John",
    age: 30
  };

  let map = new Map(Object.entries(obj));

  alert( map.get('name') ); // John
}

{
  let prices = Object.fromEntries([
    ['banana', 1],
    ['orange', 2],
    ['meat', 4]
  ]);

  // now prices = { banana: 1, orange: 2, meat: 4 }
  alert(prices);
  alert(prices.orange); // 2
}

{
	let map = new Map();
  map.set('banana', 1);
  map.set('orange', 2);
  map.set('meat', 4);

  let obj = Object.fromEntries(map.entries()); // make a plain object (*)

  // done!
  // obj = { banana: 1, orange: 2, meat: 4 }

  alert(obj.orange); // 2
}