Equality check between two sparse arrays iterate over holes to compare them

[dubzzz]

🐛 Bug Report

Checking the equality of two sparse arrays seems to iterate over all the cells of each of those (as soon as they have the same size).

By looking at the code I found that there was a special case for sparse arrays: https://github.com/facebook/jest/blob/0a9e77d5e75adc87f8d7497ec30472c661c6fc11/packages/expect/src/utils.ts#L334

But its seems that the special case only checks hole versus undefined case as it finally calls equals that will iterate over all indexes one after the other: https://github.com/facebook/jest/blob/0a9e77d5e75adc87f8d7497ec30472c661c6fc11/packages/expect/src/jasmineUtils.ts#L153

I am wondering if we could not have it faster. Actually for sparse arrays iterating over the keys and not all the indexes should be enough to confirm equality, no?

To Reproduce

Steps to reproduce the behavior:

expect(Array(100000000)).toEqual(Array(100000000)); // or longer

Expected behavior

Ends quickly: lengths are equal, keys are equal, values associated to keys are equal.

envinfo

  System:
    OS: Windows 10 10.0.19042
    CPU: (8) x64 Intel(R) Core(TM) i7-10510U CPU @ 1.80GHz
  Binaries:
    Node: 14.15.4 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.5 - ~\.yarn\bin\yarn.CMD
    npm: 6.14.10 - C:\Program Files\nodejs\npm.CMD
  npmPackages:
    jest: ^26.1.0 => 26.6.3

Context

I am currently working into adding generators for sparse arrays in fast-check (see dubzzz/fast-check#1447). My plan was to provide such helper built-in as I believed it could help to detect some bugs...

[dubzzz]

@SimenB If I'm not wrong I think the only remaining part for this issue was the fact that the computation of the stringified version of an Array(100000000) is taking lots of time in Jest.

Indeed Jest creates something like [,,,...,,,]. But it sounds expected if we want this string version to be printed on user side. On fast-check when arrays are pretty sparse (with many holes) I print something like Object.assign(Array(100000000), {5: 'value for 5'}) but it's not perfect as it seems less readable than a [,,,....,,,,].

Not sure 100% but if I'm not wrong the stringified version is computed even if the assertion passes. But I'm really not sure about that last statement.

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