toStrictEqual does not cover sparse arrays

[marcfallows]

I'm getting what I consider unexpected behaviour for toStrictEqual, treating sparse arrays incorrectly:

expect([undefined, 1]).toStrictEqual([, 1]); // will pass :(

I would expect this assertion to fail.

While in terms of index access it seems these arrays are equivalent (because [,1][0] === undefined), sparse arrays do behave differently:

> [undefined, undefined, undefined, 1].forEach(x => console.log(x))
undefined
undefined
undefined
1

> [, , , 1].forEach(x => console.log(x))
1

This can also be seen as follows:

Object.keys([undefined, undefined, undefined, 1]); // ["0", "1", "2", "3"]
Object.keys([,,, 1]); // ["3"]

And so I don't think it's appropriate to consider these strictly equal.

Happy to contribute a fix, but just want to verify that this would be something you agree is a bug.

[jeysal]

I agree this is a bug, especially since the docs say "Keys with undefined properties are checked. e.g. {a: undefined, b: 2} does not match {b: 2} when using .toStrictEqual." and do not state that it might work differently for arrays.
cc @SimenB @thymikee

[jeysal]

@emilsjolander (contributed toStrictEqual originally) do you agree?

[emilsjolander]

Agreed, this was an oversight

[jeysal]

@marcfallows sounds like a nicely isolated bug for a first Jest PR :)

[marcfallows]

PR raised: #7591

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