repl: Failed assignment "blocks" further assignments to a variable name

[dodev]

Hi!

On an assignment statement TypeError, using the same variable name is not possible due to either Reference or TypeError when trying to assign a value to a variable with that name.

Steps:

1. Assign to a variable with statement which causes TypeError ((let|const|var) a = Math.PI())
2. Try to assign to the same variable name using let|const|var OR try to use the variable;

Result:
The variable name is no longer available, because you get a TypeError: Identifier has already been declared OR ReferenceError: variable is not defined if you try to use the variable name

Expected Behavior:
The variable name to remain undeclared.

• Version: v4.4.2, v5.10.1
• Platform: Linux 4.4.0-1-amd64 deps: update openssl to 1.0.1j #1 SMP Debian 4.4.6-1 (2016-03-17) x86_64 GNU/Linux
• Subsystem: repl

Sample Output:

> let c = 1;
undefined
> let a = c();
TypeError: c is not a function
    at repl:2:9
    at REPLServer.defaultEval (repl.js:269:27)
    at bound (domain.js:287:14)
    at REPLServer.runBound [as eval] (domain.js:300:12)
    at REPLServer.<anonymous> (repl.js:439:12)
    at emitOne (events.js:95:20)
    at REPLServer.emit (events.js:182:7)
    at REPLServer.Interface._onLine (readline.js:211:10)
    at REPLServer.Interface._line (readline.js:550:8)
    at REPLServer.Interface._ttyWrite (readline.js:827:14)
> let a = 1;
TypeError: Identifier 'a' has already been declared
    at repl:1:1
    at REPLServer.defaultEval (repl.js:269:27)
    at bound (domain.js:287:14)
    at REPLServer.runBound [as eval] (domain.js:300:12)
    at REPLServer.<anonymous> (repl.js:439:12)
    at emitOne (events.js:95:20)
    at REPLServer.emit (events.js:182:7)
    at REPLServer.Interface._onLine (readline.js:211:10)
    at REPLServer.Interface._line (readline.js:550:8)
    at REPLServer.Interface._ttyWrite (readline.js:827:14)
> a = 1;
ReferenceError: a is not defined
    at repl:1:3
    at REPLServer.defaultEval (repl.js:269:27)
    at bound (domain.js:287:14)
    at REPLServer.runBound [as eval] (domain.js:300:12)
    at REPLServer.<anonymous> (repl.js:439:12)
    at emitOne (events.js:95:20)
    at REPLServer.emit (events.js:182:7)
    at REPLServer.Interface._onLine (readline.js:211:10)
    at REPLServer.Interface._line (readline.js:550:8)
    at REPLServer.Interface._ttyWrite (readline.js:827:14)
> 

[Fishrock123]

The current behavior is technically semantic, but this is a repl so I agree it is somewhat poor usability.

Ideally if you did const x; it would delete the existing x, even if it was const, and make the new declaration.

I'm not actually sure this can be fixed though, because I'm pretty sure it's interpreted to the VM like a script. 😕

[jasnell]

+1 this has been somewhat of an irritation for me for a while. Just not convinced that it is something that can be fixed in Node.js as I believe it's a v8 issue (Chrome gives precisely the same error). Quick test in the Chrome dev console shows:

const m = n
VM192:1 Uncaught ReferenceError: n is not defined(…)
const m = 1
VM221:1 Uncaught TypeError: Identifier 'm' has already been declared(…)

[Fishrock123]

It originates in v8 I think, but I doubt it is an "issue" on that level, it's just how the language semantics work.

Maybe we could give short errors for redeclarations in the REPL along with a more informative message.

[dodev]

@jasnell In chromium's repl I get a different behavior, than the one I get in node. But it is unexpected non the less:

> const q = 1();
VM481:2 Uncaught TypeError: 1 is not a function(…)(anonymous function) @ 
> const q = 1;
undefined
> q
VM520:2 Uncaught ReferenceError: q is not defined(…)(anonymous function) @ 

[jasnell]

Sigh.. gotta love consistency. +1 to seeing if we can at least improve the errors in REPL.

[lance]

This seems to be a side effect of the way Javascript evaluates let and const declarations as defined by the spec: http://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations. Based on this grammar, I can see why const would behave this way, since the variables are "created when their containing Lexical Environment is instantiated". So for const, it seems the variable is created, but when the exception is thrown, it's the lingering const reference cleaned up. But I'm kind of spitballing here, since let variables can be reassigned, and so should not throw a ReferenceError when reassigned.

Without digging too much, it almost seems like a V8 bug to me, but a quick search of the bug tracker doesn't shed any light: https://bugs.chromium.org/p/v8/issues/list?can=1&q=assignment+exception&colspec=ID+Type+Status+Priority+Owner+Summary+HW+OS+Component+Stars&x=priority&y=owner&cells=ids

[jasnell]

Yep, there's actually nothing we can do on this one in Node except maybe make the error a bit more informative. There has been at least one other issue about this reported. Going to close this as it's not something we can fix.