Lexical variable declarations (`let`, `const`) unusable in repl if error thrown during declaration/first assignment

[polybuildr]

• Version: v.6.3.1
• Platform: Linux (Ubuntu 14.04)
• Subsystem: repl (I think)

let s = Set();

gives TypeError: Constructor Set requires 'new'.

However, after this:

s

gives ReferenceError: s is not defined

and

let s = new Set();

gives TypeError: Identifier 's' has already been declared.

I'm not sure whether this is a real bug or it's expected behaviour, but it sure does seem unusual that s becomes unusable from this point in in the repl.

This isn't a problem when running node on a js file because the TypeError simply terminates execution.

[vsemozhetbyt]

Different with var:

> var  s = Set();
TypeError: Constructor Set requires 'new'
> s
undefined
> var  s = new Set();
undefined
> s
Set {}

[polybuildr]

Yep, works fine with var.

[vkurchatkin]

Here is simplified test case:

var vm = require('vm');

try {
  vm.runInThisContext('let x = f');
} catch (e) {}

vm.runInThisContext('x = 1');

[addaleax]

This is very déjà-vu-ish to me, although I can’t find another issue for this bug… /cc @nodejs/v8?

[polybuildr]

This error also occurs in the Chrome browser, so it's probably a v8 thing and not a Node thing. Sorry for reporting it in the wrong place.

[polybuildr]

Where should I report this considering that it's not specific to node's repl?

[Fishrock123]

I wouldn't be surprised if this was how the spec worked.

In general, you'll have a better time using var for variables in the repl.

[bnoordhuis]

@polybuildr You can report it over at https://bugs.chromium.org/p/v8/issues/list. It looks spec compliant to me (the binding for s is never fully constructed and stays in the TDZ) but perhaps the error messages can be improved.

[Slayer95]

This is per spec: https://esdiscuss.org/topic/take-let-variable-out-of-temporal-dead-zone

[polybuildr]

@bnoordhuis, @Slayer95, it does look spec-compliant, so I'm going to close this issue. However, that esdiscuss topic pointed me to something interesting that the Firefox Devtools console does and might be worth a shot.

For any such unresolved bindings (at least in the DevTools), Firefox steps in and turns them into undefined. So a redeclaration with let doesn't work, but simply s = 5 works from that point on. If anyone would like to consider doing this for the repl, feel free to reopen the issue.