[quarterto]

Have you read the linked article about typeof? Normally, it's safe to do typeof possiblyUndeclaredVariable, but if later in the function you do let possiblyUndeclaredVariable it starts the scope defined as "uninitialized", which causes typeof to throw.

[umurkontaci]

Well that's perfectly normal in a properly scoped language.

[virmundi]

What you have to keep in mind is that JavaScript is not "a properly scoped language". Pretending that it is will cause you to miss key aspects of how the tool works. This helps no one, including you. Please, for as crappy as the language might feel, approach the language on its own terms.

[umurkontaci]

Function scopes and variable hoisting was not the best parts of JS anyway. let brings lexical scope into the game, which I think is a great progress. And you cannot expect a variable to be defined outside of its lexical scope. That's not any different that trying to access a variable outside of a function that is defined in.

If people were abusing variable hoisting in some way, they can continue to do so, by not using `let`.

[makomk]

If I'm following the blog post correctly, let does have effects outside of its lexical scope: it effectively makes the variable even more undefined than a totally non-existent variable. That is, the following code will not throw anything:

    // x has not been defined or initialized anywhere
    console.log(typeof x)

However, if you add a let statement after it like so:

    // x has not been defined or initialized here
    console.log(typeof x)
    let x = "foo";

typeof will fail with an exception, because the let statement changes x from an undefined variable to a new state that isn't even undefined anymore.

[ajanuary]

It's not having an effect outside of its lexical scope. You introduce the let into the same lexical scope as the console.log.

It will also throw an exception every single time. So unless you're adding a variable, and then never testing the code path that hits the new line, you're probably going to catch it pretty early.

[TeMPOraL]

Well, but doesn't let itself create a new implicit scope, from the point of the let statement to the end of the scope the statement is in? It seems to me that here let is indeed acting on things outside of lets own implicit lexical scope.

[umurkontaci]

I understand, but I believe this still happens only within the lexical scope where the variable has been defined with let.

TBH, It doesn't bother me at all. I think I can even go ahead and say the latter makes much more sense. Using the former one is abusing the weaknesses of the language that has come along with it throughout its history.

We have been whining about the bad parts of JavaScript for a long, long time and I think these changes are for the better for all of us.

[TeMPOraL]

> I understand, but I believe this still happens only within the lexical scope where the variable has been defined with let.

Doesn't let itself introduce a new, implicit, scope? Here it seems let acts on things outside its own scope, which seems at least ugly.

[jrochkind1]

> And you cannot expect a variable to be defined outside of its lexical scope.

Okay, but since `typeof x` can return "undefined", I would expect it to return `undefined` if x is no defined. Now, sometimes `typeof x` returns `undefined` if x is not defined, but other times it raises if x is no defined. That's the issue.

I too predict this is going to give people a lot of trouble. We will see.

[TeMPOraL]

And the worst thing is, introducing a let variable affects code outside of the implicit lexical scope created by that let. The switch from "undefined" to "more undefined" happens one scope above. Even if it doesn't create much of a trouble for people, it feels like a very ugly design.

[ajanuary]

typeof isn't raising the exception, the runtime is raising it before typeof is even evaluated.

[jrochkind1]

I understand the implementation details, but the fact remains that previously you could call `typeof something` for any `something` at all, and it would never throw, ever. So you could use it as a way to check if a variable was defined.

Now, sometimes it will throw.

I mean, come on, you really don't see anything confusing in the fact that there are now two kinds of `undefined`, one kind you can call `typeof x` and it returns `undefined`, but another kind of undefined where you can't mention x at all without a throw? Like, if you can't mention undefined variables without getting a throw, how come `typeof` sometimes returns `undefined`? Ah, because some "undefined" variables you can do that with, but not others? So I guess there is more than one "kind" of "undefined" now? This is not confusing?

[the8472]

>it's safe to do typeof possiblyUndeclaredVariable

What would be the usecase of typeof'ing something undeclared in a block scope and then declaring the let further down?

Even more so with strict mode which does't really allow you to use undeclared variables anyway.