Immutable means unchangeable i.e. you can't change.
Javascript has lots of immutable types e.g. string primitive type. Try this in your console.
s = "red";
console.log(s[1]); //→ "e"
s[1] = "x";
console.log(s) //→ "red"the s didn't change! WAT!
Details
Some string methods like String.replace returns a new string.
JavaScript has one complex data type, the Object data type, and it has five simple data types: Number, String, Boolean, Undefined, and Null. These simple (primitive) data types are immutable (cannot be changed), while objects are mutable (can be changed).
tags: js, immutability