What's the difference between using let and var in JavaScript?

When should you use let over var? And why? Let's find out!

In modern JavaScript we have 3 ways to declare a variable and assign it a value:

• const
• let
• var

When working with variables in JavaScript, I always default to using const. It guarantees the value can’t be reassigned, and so it’s safer to use.

But when I do need to redeclare a variable later on, I always use let.

I haven’t used var in years, and to me it’s just there for backwards compatibility purposes, and I always raise an eyebrow when I see it used.

Why?

First, let has sensible scoping.

The same scoping that is used in more or less all popular programming languages, block scoping, dictates that variables declared using let are scoped to the nearest block.

var instead is a bit more weird, as it has function scoping, which means that variables declared using var are scoped to the nearest function.

This has practical implications. For example, a variable is declared inside an if or used as the for loop iterator. Using let makes it local to those 2 blocks. Using var, however, allows the variable to be available outside of that block, which might lead to bugs.

Always use the tool that gives you the least amount of power, to make sure you have maximum control over it. With great power comes great responsibility.

Another reason to prefer let is hoisting. Like const, let variables are not hoisted, but they are initialized when evaluated.

var variables instead are hoisted to the top of the function, and as such they are available even in the lines before their declaration. Weird, right?

Third reason: when you declare a let variable with the same name as one that already exists, you get an error (in Strict Mode).

Finally, another big difference: if you declare a var variable outside of any function, it’s assigned to the global object, which means window inside the browser. let does not work in this way; the variable is available, but not attached to the global object, and so it’s not reachable from outside of your file.