Checking whether an array's constructor is === Array fails

[ZeroXLR]

• Version: v6.3.1
• Platform: Darwin xxxxx.local 15.6.0 Darwin Kernel Version 15.6.0: Thu Jun 23 18:25:34 PDT 2016; root:xnu-3248.60.10~1/RELEASE_X86_64 x86_64

Problem Statement

The simplest instance of the problem is this: I have a file, say test.js in my Desktop, containing the following code:

module.exports = function(a) {
    return a.constructor === Array;
};

When I run this in the node REPL, it gives me an odd result:

xxxxx@xxxxx:~/Desktop$ node -v
v6.3.1
xxxxx@xxxxx:~/Desktop$ node
> var t = require('./test.js');
undefined
> t([1, 2]);  <<<--- Passing in [1, 2] whose constructor should be Array
false          <<<--- What?!
> [1, 2].constructor === Array;
true
> var tprime = function(a) {return a.constructor === Array;};
undefined
> tprime([1, 2]);
true

Since t is simply checking whether the constructor of its argument is Array, I was expecting a true when you passed in [1, 2] whose constructor is, in fact, an Array.

Running Problem Code against LTS

I ran the exact same test after uninstalling v6.3.1 and installing v4.4.7 and the problem disappears:

xxxxx@xxxxx:~/Desktop$ node -v
v4.4.7
xxxxx@xxxxx:~/Desktop$ node
> var t = require('./test.js');
undefined
> t([1, 2]);
true          <<<--- As expected
> [1, 2].constructor === Array;
true
> var tprime = function(a) {return a.constructor === Array;};
undefined
> tprime([1, 2]);
true
> 

Checking against String

The problem also disappears if I change test.js to check against String instead of Array (in both v6.3.1 and v4.4.7). Here is the new test.js:

module.exports = function(a) {
    return a.constructor === String;
};

And here are the checks in REPL:

xxxxx@xxxxx:~/Desktop$ node -v
v6.3.1
xxxxx@xxxxx:~/Desktop$ node
> var t = require('./test.js');
undefined
> t("sdfdsf");
true          <<<--- No issue this time
> "sdfdsf".constructor === String;
true
> .exit
xxxxx@xxxxx:~/Desktop$ node -v
v4.4.7
xxxxx@xxxxx:~/Desktop$ node
> var t = require('./test.js');
undefined
> t("sdfdsf");
true
> "sdfdsf".constructor === String;
true
> .exit
Veritas@nfr:~/Desktop$

Conclusion

Thus, in conclusion, is there a real bug here or is this a known valid difference between Node versions v4.x.x and #v6.x.x?

[addaleax]

See #7788, this will be fixed in the next v6.x release. I’m closing this as a duplicate, but feel free to ask follow-up questions!

[ZeroXLR]

Glad to get a quick reply. I have no further questions.