Map objects are collections of key-value pairs. A key in the Map may only occur once; it is unique in the Map‘s collection.

Table of Contents

Main methods

Method	Description
`new Map()`	creates a new Map object
`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 element (the key/value pair) by the key
`map.clear()`	removes everything from the map
`map.size`	returns the current element count

Ex:

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 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

Note:
Although map[key] also works, e.g. we can set map[key] = 2, this is treating mapas a plain JavaScript object, so it implies all corresponding limitations (only string/symbol keys and so on).
So we should use mapmethods: set, getand so on.

Map can also use objects as keys.

Ex:

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 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

Instance methods

Method	Description
`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`

Ex:

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)
}

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)

}

Objects vs Maps

Map	Object
Directly iterable	Not directly iterable
Have a size property	Do not have a size property
Keys can be any datatype	Keys must be Strings (or Symbols)
Keys are ordered by insertion	Keys are not well ordered
Do not have default keys	Have default keys