feat(flatMapDeep): Add flatMapDeep

[ssi02014]

cloesd: #347

Add the flatMapDeep discussed in the issue above.

benchMark

This is more performant than utilizing lodash or the built-in array functions.

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[ssi02014]

@raon0211 As a footnote to that function, the return type of flatMap is Array<FlatArray<U[], D>>, so specifying the type as infinity will not result in the intended type inference.

Therefore, we believe that flatMapDeep with the intended type inference is useful and serves the purpose of compatibility with lodash.

const arr = [1];

const newArr1 = flatMapDeep(arr, item => [[[item, item, [[item]]]]]); // number[]

newArr1.map(item => {
  item; // number
  item.toFixed(1); // ✅
});

const newArr2 = flatMap(arr, item => [[[item, item, [[item]]]]], Infinity);
// FlatArray<(number | number[][])[][][], 0 | 1 | -1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20>[]

newArr2.map(item => {
  item; // FlatArray<(number | number[][])[][][], 0 | 1 | 2 | 3 | 4 | -1 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20>

  item.toFixed(1); // ❌
  (item as number).toFixed(1); // ✅ But this is not comfortable....
});

The issue with the Infinity type in typescript was previously mentioned by @raon0211 himself. https://github.com/toss/es-toolkit/issues/122#issuecomment-2214201240

Of course, we could work around it with something like flatMap(arr, 9999), but I think it's better to provide flatMapDeep than to do that.

const newArr3 = flatMap(arr, item => [[[item, item, [[item]]]]], 9999); 
// ..🤔 I don't think this is appropriate.

newArr3.map(item => {
  item; // number
  item.toFixed(1); // ✅
});

In fact, the performance difference between the two functions is not significant.

[raon0211]

Yes, you're right. I accidentally forgot what we discussed 😅