Array-Object-Map-Set.md

Array

In JavaScript, every Objects including Arrays are passed by reference

let A = [1,3,5];
let B = A;
A.splice(1,1,10);
what is B ? // B is also [1,10,5] because assigning arrays works as pass by reference

A.length=0;
What is B ? // B is empty array [] because array A is []

Map

Difference between Map and Object

Map is similar to Object but with a few more features. Map is similar to associative arrays in PHP.

1. Map can have keys of any type whereas Object can have only string keys. Map can have a string, number, boolean or even an object as a key. But, Object would convert anything into a string as key. So for eg:

//Demonstration of object key for an Object
let john = {
  name: 'john',
  id: 1,
};

let O1 = {};
O1[john] = 100;
// the key john would become as {[object Object]: 100}

// Demonstration of object key for a map
let map = new Map();
map.set(john, 100);
console.log(map.get(john)); // 100

2. Map iterates in the same order as how elements are inserted whereas for objects there is no order.

3. Map has size property but, Object doesn't have size property

Convert an Object to a Map

let fruits = {
  apple: 1,
  banana: 5,
  orange: 3,
};
let fruitsMap = new Map(Object.entries(fruits));
fruitsMap.get('banana'); // 5

Convert a Map to an Object

let fruitsMap = new Map([
  ['apple', 1],
  ['banana', 5],
  ['orange', 3],
]);

let fruits = Object.fromEntries(fruitsMap.entries());

Set

Set is similar to Map with the following major changes.

• A Set has a unique collection of items. Even if you add same value multiple times, set adds that value only once.
• There is no keys for Set

let set = new Set([iterable]);
let set = new Set(Object.values(fruits));
let set = new Set(myArray);