Map

This is an object that holds key-value pairs (similar to a Dictionary if you know C#).

Let's create a map object. This will contain fruit types and the associated quantity in kg for a store.

var store = new Map();

Adding items

To add items, we have the set() function to which we pass a key and a value to store:

store.set('peach', 34);
store.set('banana', 14);
store.set('lemon', 10);
store.set('strawberry', 22);

WARNING

It is important to say that map maintains/keeps the order in which you add items (unlike objects).

Let's display the items as they are to the console:

for (const item of store) {
  console.log(item);
}

const

Notice the usage of const here: it creates a new variable with each iteration.

So we don't reassign, we create a new one.

This prints out:

Array [ "peach", 34 ]
Array [ "banana", 14 ]
Array [ "lemon", 10 ]
Array [ "strawberry", 22 ]

If you want to show only the keys, you can then call the keys() function:

for (const item of store.keys()) {
  console.log(item);
}

peach
banana
lemon
strawberry

Also, we have the ability to display only the value by calling the values() function:

We can get the size of a map through its size attribute:

console.log(store.size); // 4

If you have a key and you want to get the item based on that key you can use the get method:

var lemonQuantity = store.get('lemon');
console.log(lemonQuantity);

You can also do declaration and initialization (to assign a value) in one statement:

var store = new Map([
  ['peach', 34],
  ['banana', 14],
  ['lemon', 10],
  ['strawberry', 22],  
]);

And this will behave the same as before:

for (const item of store) {
  console.log(item);
}