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Immutability in JavaScript

Today, I wanted to take a quick look at the concept of immutability in JavaScript (or it’s lack thereof), and its implication on how you write code.

What is “immutability”

In JavaScript, things that are immutable don’t change in value when you use them, and things that are mutable do.

Let’s look at some examples.


First up, strings. Strings are immutable. In the example below, setting a new variable of str2 by calling slice() on str1 in no way changes the value of str1;

var str1 = 'Hello, there!';
var str2 = str1.slice(0, 5);

// logs "Hello, there!"

// logs "Hello"

You can still change the value of str1.

str1 = 'Goodbye, friend!';

But the value of the the original string itself doesn’t change when do things with it.


Numbers are also immutable. Doing some math on the num1 variable and setting it to a new variable doesn’t change the value of num1.

var num1 = 42;
var num2 = num1 - 7;

// logs 42

// logs 35

This all seem painfully obvious, right? Then we get to arrays and objects.


Arrays are mutable.

In the example below, we’re setting the variable arr2 to the value of arr1, and then pushing a item, cookie, into it.

Surprisingly, the value of arr1 changed, too! In this example, arr2 isn’t a new array the way referencing strings and numbers would be. It’s the original array pointed to by another variable.

var arr1 = ['sandwich', 'soda', 'chips'];

var arr2 = arr1;

// logs ["sandwich", "soda", "chips", "cookie"]

// logs ["sandwich", "soda", "chips", "cookie"]


Objects are also mutable.

Here, we’re setting the value of obj2 to obj1, and then adding a new item with a key of cookie and value of chocolate chip to it. But when you log both objects, they have all of the same properties!

var obj1 = {
	sandwich: 'turkey',
	soda: 'Pepsi',
	chips: 'Cape Cod'

var obj2 = obj1;
obj2.cookie = 'chocolate chip';

// logs {sandwich: "turkey", soda: "Pepsi", chips: "Cape Cod", cookie: "chocolate chip"}

// logs {sandwich: "turkey", soda: "Pepsi", chips: "Cape Cod", cookie: "chocolate chip"}

The challenge with mutability

Often, when setting a new array or object and referencing the value of an exiting one, you’re trying to create a new item and preserve the value of the original.

The mutability of arrays and objects makes it harder to trust the original source of data.

Tomorrow, we’ll look at some strategies for dealing with mutability.