Differences in objects with properties that are associative arrays are not detected about react-fast-compare HOT 8 CLOSED

I'm assuming this is because of the odd way that JS returns a length of 0 for associative arrays.... but perhaps this is intended behavior?

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Can you show us how you are creating those two sample objects in code? And perchance do you know how lodash's isEqual treats them?

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Sure thing... here's how I created a:

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Not sure how lodash treats the same... but I will add in the library and attempt the same if you'd like.

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Yeah, so I did this experiment, and lodash (which is generally considered the "most correct" although definitely not the fastest deep compare) behaves similarly:

In RFC's case, we rely on Array.isArray() returning true to allow us to just iterate and compare items by length and index up to length. (See https://github.com/FormidableLabs/react-fast-compare/blob/master/index.js#L18-L24 for full implementation). I'm guessing lodash does something similar, as to accommodate the non-standard use case of manually added properties to an object of type Array would need to be something like:

1. First, treat as array -- Iterate i up to .length
2. Second, if all true, treat as object -- iterate Object.keys.

... and that's going to have perf implications that we'd really want to balance with a large user-base need. Perhaps you can chat more about how first-class Arrays are coming up with custom, object-style, fields on them to help frame the discussion better? Thanks!

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Interesting... and also makes sense performance-wise.

For my particular use case, switching from the use of an Array to Object to store individual keyed values solved the problem without any need for major refactoring. This is from an older codebase I'm refactoring, so the use of Arrays in this manner is a side-effect of not-so-clean legacy code.

I think it's still important bring up the issue in case anyone else is experiencing the same issue with code that utilizes Arrays in this manner.

Thank you much for verifying and for the detailed response and explanation!

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Hi, is it possible that the same issue can apply to array of objects that have different property values?

For example:

[
  {
    filterValue: undefined
  }
]

[
  {
    filterValue: "a"
  }
]

I have a case where I am comparing react props and the changes that occur are in a nested array object's property value, it doesn't seem to detect the change, unless I reduce the data to the specific objects.

Could it be a similar issue to the above?

Sorry if I am being unclear.

Update: ok I think I know my issue, its because some props have circular references.

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Closing this, as it appears to be resolved/addressed.

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