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Add types for Object.groupBy() and Map.groupBy() #56805
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1bef967
Add types for Object.groupBy() and Map.groupBy()
bakkot ce78dff
fix formatting
bakkot d402112
rebaseline
bakkot 547e844
fixup: add more coauthors
bakkot 53ac772
rename and reorder type parameters
bakkot 3e53691
rebaseline
bakkot c4fb034
Merge branch 'main' into groupby
bakkot ee4a13c
rebaseline
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,11 @@ | ||
interface MapConstructor { | ||
/** | ||
* Groups members of an iterable according to the return value of the passed callback. | ||
* @param items An iterable. | ||
* @param keySelector A callback which will be invoked for each item in items. | ||
*/ | ||
groupBy<K, T>( | ||
items: Iterable<T>, | ||
keySelector: (item: T, index: number) => K, | ||
): Map<K, T[]>; | ||
} |
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,11 @@ | ||
interface ObjectConstructor { | ||
/** | ||
* Groups members of an iterable according to the return value of the passed callback. | ||
* @param items An iterable. | ||
* @param keySelector A callback which will be invoked for each item in items. | ||
*/ | ||
groupBy<K extends PropertyKey, T>( | ||
items: Iterable<T>, | ||
keySelector: (item: T, index: number) => K, | ||
): Partial<Record<K, T[]>>; | ||
} |
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why is the return type
Partial
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Because you might not actually get all of the keys.
As a trivial example, consider the case that the iterable is empty. Then the resulting record will have no keys at all. Typing as
Record<K, T[]>
would mean that e.g.Object.groupBy(vals, x => x < 5 ? 'small' : 'large')
would be typed as always havingsmall
andlarge
keys, such thatresult.small.length
would check without errors, which is misleading.There was a problem hiding this comment.
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Ah, right, I forgot that
Map<K, T[]>
hasget: (k: K) => T[] | undefined
so was confused by the difference in types.