Practice: Immutable vs. Mutable in JavaScript

Primitive

In JavaScript, a primitive (primitive value, primitive data type) is data that is not an object and has no methods. There are 7 primitive data types: string, number, bigint, boolean, undefined, symbol, and null.

Most of the time, a primitive value is represented directly at the lowest level of the language implementation.

All primitives are immutable, i.e., they cannot be altered. It is important not to confuse a primitive itself with a variable assigned a primitive value. The variable may be reassigned a new value, but the existing value can not be changed in the ways that objects, arrays, and functions can be altered.

Source: Mozilla, https://developer.mozilla.org/en-US/docs/Glossary/Primitive
This work is licensed under a Creative Commons Attribution-ShareAlike 2.5 License.

Example

This example will help you understand that primitive values are immutable. Start by opening a Developer Console in your browser. The sequence is ctrl+shift+J on Windows and cmd+option+J on the Mac. Then, you'll use the "console.log" method to write output to the developer console.

JavaScript

// Using a string method doesn't mutate the string
var bar = "baz";
console.log(bar);               // baz
bar.toUpperCase();
console.log(bar);               // baz

// Using an array method mutates the array
var foo = [];
console.log(foo);               // []
foo.push("plugh");
console.log(foo);               // ["plugh"]

// Assignment gives the primitive a new (not a mutated) value
bar = bar.toUpperCase();       // BAZ

A primitive can be replaced, but it can't be directly altered.