Constant or const when to use in JavaScript?

According to ES6 specification a constant:

  • cannot change through re-assignment
  • cannot be re-declared

If we declare an object as constant then we can add properties to that object. Similarly when we declare an array as constant then we can add items to the array list.

In the case of an array or object, we are not re-assigning or re-declaring the constant. The constant is already declared and assigned, we are just adding to the “list” that the constant is pointing to.

Object example

const obj = {};

obj.firstname = 'Joe';

console.log(obj); // {firstname : 'Joe'}

obj.lastname = 'Smith';

console.log(obj); // {firstname : 'Joe', lastname: 'Smith'}

Array object example

const arrList = [];


console.log(arrList); // ['Joe']


console.log(arrList); // ['Joe', 'Smith']


console.log(arrList); // ['Joe']

Not possible with constant

const obj = {};
obj = {firstname: 'Joe'}; // error - re-assigning

const arr = ['Jennifer'];
const arr = ['Joe']; // error - re-declaring

const arrList = 'Joe';
arrList = 'Jennifer';       // error - can not re-assign
var arrList = 'Hello';   // error - already declared
function arrList() {};  // error - already declared