variable is immutable code example

Example 1: A mutable object

Mutable is a type of variable that can be changed. 
In JavaScript, only Objects and Arrays(technically a type of object) are 
mutable, not primitive(immutable) values.

Immutables are the objects whose state cannot be changed once the object is 
created. Boolean, Null, Undefined, Number, String, and Symbols are immutable.

Example 2: what is immutable

// by negative example here is how to make a javascript immutable

'use strict';

const a = Object.freeze([4, 5, 6]);

// Instead of: a.push(7, 8, 9);
const b = a.concat(7, 8, 9);

// Instead of: a.pop();
const c = a.slice(0, -1);

// Instead of: a.unshift(1, 2, 3);
const d = [1, 2, 3].concat(a);

// Instead of: a.shift();
const e = a.slice(1);

// Instead of: a.sort(myCompareFunction);
const f = R.sort(myCompareFunction, a); // R = Ramda

// Instead of: a.reverse();
const g = R.reverse(a); // R = Ramda