Is there a reason not to replace `var` with `let` in JavaScript? [duplicate]

Question

A question for my elders in JavaScript: putting aside the fact that let has a different variable scope than var does, it seems to me that in the vast majority of cases, there is no reason to use var over let. Of course, if you define a variable inside of an if statement, it's not the same thing, but is there another reason besides scope to use one over the other?

Edit: To be clear, this question is about a best practice (i.e. pros and cons) in JavaScript, not about the nature of the differences between var and let.

Thanks!

Answer 1

No, I think there are no reasons. From what I've read, the var is left in ES6 for backwards compatibility. Most of the articles I've read advise to gradually replace all var occurrences with let. Of course, this should be done accounting for differences such as the one you mentioned with if block. I use only let in my ES6 code now. Eric Elliott doesn't use var either. There’s nothing using var that let can’t do better. If you need entire function scoping, create the let at the top of the function.

There is one difference that I've noticed. It is that when defining variables in the global scope, variables defined as var are accessible with window.variable, while variables with let are not accessible using window object. But with the properly written modular code, no var statement should be allowed to leak to window and so this has no importance.

I would also advise to use const instead of let for the cases when a variable reference (as opposed to a primitive variable's value) will not be mutated.