Definition

Mutable means being subject to change or alteration, on the other hand Immutable is the opposite (cannot be changed)

Mutables

In Javascript Objects and Arrays are mutable, which means after creating one we can modify them later in our code. Javascript Mutable Types Example

Immutables

That was the case for Objects and arrays, for the primitive data types (string, number, boolean, null, undefined, symbol) they cannot be altered (Immutable). Javascript Immutable Types Example