A case for prefer-flat-map

[tinovyatkin]

I think it will be way greater rule if it will be able to identify legacy (before .flat got supported) code and suggest to refactor to flatMap. I believe one of the most common scenario was use of [].concat with array spread:

[].concat(...someArray.map(fn))
// is the same
someArray.flatMap(fn)

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Backers (Total: $40.00)

• issuehunt ($40.00)

Submitted pull Requests

• #323 prefer-flat-map: Add warning for common concat pattern

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[sindresorhus]

Good idea!

[fisker]

should we consider Symbol.isConcatSpreadable?

[sindresorhus]

@fisker Consider it how? Can you elaborate?

[fisker]

@sindresorhus

this demo seems not working on my chrome, but works on firefox

check this https://jsbin.com/farogop/edit?js,console

though this maybe not very common case,
I'm just saying, It's possible not the same as flatMap, and we can't guarantee new Objects not ConcatSpreadable never come in future

[MrHen]

@fisker I'm comfortable using // eslint-disable-next-line prefer-flat-map to ignore cases like that.

[sindresorhus]

@fisker That's too much of an obscure use-case to handle.

[fisker]

Imaging this fn is from a third-party library, which is isConcatSpreadable false

[].concat(...someArray.map(fn))

When you wrote and tested, It works as expected, you may not think about adding // eslint-disable-next-line prefer-flat-map.
When you format your code and publish, It breaks. You have to debug your program to locate the problem.

If a linter can't be safe, what's the point of using it.

I insist not to fix this code, at least an option, people should know this may cause problems

[sindresorhus]

@fisker This seems very unlikely, but sure, we can omit fixing this case.

If a linter can't be safe, what's the point of using it.

Then you should probably not use JavaScript. Most lint rules are not 100% safe. That is just impossible for such a loose language as JS.

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