Const, let and var, which and when?

Another article on JavaScript variable assignment 👍

Odds are that you’ve seen the “new” variable assignment keywords const and let. They’ve been around a while now 😅

Support for them is good now and if you use a transpiler such as babel, you’ve likely been using them for some time.

For those in camp TL;DR; If in doubt sort priority of use alphabetically, const before let, let before var. Prioritise read-only variables within your code. This reduces the risk of accidental reassignments and unintentional value changes. As a bonus, if you use eslint, you can use the prefer-const rule to help you out! ⛑

var

Before transpilers and fancier syntax, there was var. var was happy go lucky. You could and still can do whatever you want with it.

Its relaxed nature can lead to issues though if you’re not familiar with how it behaves in certain scenarios.

The problem with var?

There is nothing wrong with using var. There are no problems with using var. However, there are some gotchas with using var.

Lack of block scoping

Variables assigned with the var keyword have functional scope. This is somewhat intuitive. The variable isn’t accessible outside of the function.

They aren’t block scoped though. That’s where things can get messy 😕 You can reassign variables within a block actually overriding parent assignments. Consider the following;

To clarify, doing this with a function wouldn’t change the value