Doubled array entry using index '' in node v8.4.0

[lucaelin]

• Version: v8.4.0
• Platform: Linux hostname 4.4.0-93-lowlatency test-require-resolve: use case insensitive compare #116-Ubuntu SMP PREEMPT Fri Aug 11 22:13:00 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

As of node v8.4.0, assigning a value to the array index '' causes a duplicate entry to be created. The duplicate disappears (and cannot be recreated) as soon as a value is assigned to a numerical index.

let x = [];
x[''] = 'foo';
console.log(x); 
// [ '': 'foo', '': 'foo' ]

console.log(x[0]); 
// undefined
x[0] = 'bar';
console.log(x);
// [ 'bar', '': 'foo' ]

x[''] = 'baz';
console.log(x);
// [ 'bar', '': 'baz' ]

[TimothyGu]

This is because of a bug when printing the array, not because the array actually has a duplicated property.

/cc @BridgeAR

[keangkai]

I think array is duplicate but I'm not sure

[mscdex]

It's not possible to have duplicate keys for normal JS objects like that (the "exception" being Map instances), so if anything there is a bug in util.inspect() as @TimothyGu mentioned.

[targos]

I'm looking at this

[targos]

Well, actually it seems to be already fixed on master. Can someone else confirm?

[benjamingr]

@targos looks fixed in master on macOS:

~/Documents/OpenSource/node [master] $ ./out/Release/node
> var a = []
undefined
> a[''] = 'foo'
'foo'
> console.log(a)
[ '': 'foo' ]
undefined
> 
(To exit, press ^C again or type .exit)
> 
~/Documents/OpenSource/node [master] $ node -v
v8.4.0
~/Documents/OpenSource/node [master] $ node
> var a = []
undefined
> a[''] = 'foo'
'foo'
> console.log(a)
[ '': 'foo', '': 'foo' ]
undefined
> 

Thanks for the report @chancho4321 this bug appears to have been fixed already - thanks anyway, reporting was the right thing to do :)

[TimothyGu]

That's great to hear! We should make sure to have a test case for it if one doesn't yet exist.

[BridgeAR]

This got fixed by 3a886ff. Thanks for the report as this is a interesting edge case. It could only happen if the array contained a empty string as key and no regular entry. As @TimothyGu pointed out it would be nice to add a additional test case.

Therefore I am adding the good first contribution label.

[mscdex]

@addaleax Shouldn't this still be tagged with 'v8.x' since that is where the issue still occurs (the aforementioned commit is not present in either the 'v8.x-staging' or 'v8.x' branches)?