JavaScript const vs let Debunked

const and let are two keywords used to declare variables in JavaScript.

Recently, there has been a debate on which keyword is better, which I believe to be absurd.

In this article, I will explain the key differences between const and let, then give my take on how both keywords should be used in JavaScript.

So without further ado, lets dive right in!

This debate originates from the “let me be” talk by Ryan Florence.

Ryan makes controversial remarks about the use of const and let, and shows a clear subjective bias towards the use of let.

I will not cover this in the article, since Jack Herrington evaluates this peculiar talk logically and objectively.

This article instead covers the fundamental differences between let and const.

This will enable you to make better choices based on the context, instead of following baseless and subjective opinions.

The key difference between let and const lies in the idea of re-assignment.

With let, you can re-assign a variable, meaning you can replace the actual reference link between the variable name to its value.

This is not possible using const.

Lets define two variables using let and const to see what I mean:

Now lets draw the reference links for both variables:

Reference links for let (unlocked) and const (locked)

Therefore, a statement like x = 2 is possible, because the reference link will change and point to 2 instead of 1.

The equivalent statement y = 2, doesn’t work since the reference link is locked in place.

While const works as expected for primitive types, e.g. number, the same can’t be said for more complex types such as arrays or objects.

Lets look at the example below:

Even though this variable is marked as const, you can still perform array operations such as push.

This is counter intuitive as const should denote a “constant”, something unassignable and immutable after definition.

But in JavaScript, the idea of mutability is independent from re-assignment.

For example, the following code works using let but not using const:

In JavaScript there are only two ways to achieve true constants:

Using our previous example, we can make the array immutable using Object.freeze:

This, however, is known as shallow freezing, which means only the first level is immutable, while the nested levels are still mutable:

The only way to get true immutability for objects with nested properties is by using immutability libraries like immutable-js.

This library contains no dependencies, and provides an array of different structures that are immutable and behave as expected.

After reading this article, you should be experts with let and const.

You can now confidently decide which keyword to use in any context.

Nevertheless, my opinion is to utilize let for situations when you need selective/iterative re-assignment, e.g. flag variables, loop counters.

I believe const should be used everywhere else to promote consistency.

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