Typing a difference(arr1, arr2, comparator) function

[tiny arrow]

Hey all, I'm trying to code a basic difference() function that can accept one of three kinds of comparators:

• Nothing (compare elements directly)
• String (key of both arr1 and arr2)
• Compare function (gets items from both arrays)

It looks something like this and I'm getting errors on using string to index the generics. Could anyone help me create a type from a key from both T and U? Thanks!

[digital moat]

matts#4412

Playground Link

Preview:ts ... )) ) }

[digital moat]

Ascor8522#7606

Playground Link

Preview:```ts
type CmpFn<T, U> = (x: T, y: U) => boolean

export function difference<T, U>(
arr1: T[],
arr2: U[]
): T[]
export function difference<T, U>(
arr1: T[],
arr2: U[],
comparator: keyof T & U
): T[]
export function difference<T, U>(
arr1: T[],
arr2: U[],
comparator: (x: T, y: U) => boolean
): T[]
...```

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[cunning lily]

multiple things

1. it's probably easier to use function overload in this case, since you already have the signatures figured out

it might be more verbose, but on the other hand it's cleaner than that previous union you had

2. also, I moved the default comparasion function to some variable, to make it a bit cleaner

3. I used a switch instead of that ternary operator, it's probably easier to see what's happening as well

4. the string type in your difference was wrong, it's not just any string you accept, but one that is a key on the objects contained in the array, so I changed it to keyof T & U

you could also set it to keyof T | U of you only want to allow for the key to be present on 1 object out of the 2, and if you don't mind comparing a value with undefined

@tiny arrow

[tiny arrow]

Ah yea, this is actually a good case for function overloading!

Also the switch (true) is some warlock sh*t

Any specific reason why the x value needs to be cast as unknown first before being compared to y?

Oh it’s because it doesn’t find any overlap between T and U as different arrays. Is casting something to unknown for comparison between generics the usual approach?

[cunning lily]

yteah, because if they are different types, like T and U, TS will consider they won't ever overloap, cuz different types

as you said

Is casting something to unknown for comparison between generics the usual approach?
not really, but sometimes it's one of the few options left when comparing elements of different types

imo it shouldn't be a TS error, but a warning

[tiny arrow]

Is there a way to use extends to guarantee the comparison is of the same type?

[cunning lily]

I guess

but I'm not sure there is a real need for generics in this case

it does not provide any additional benefits

you could just type both your arrays as unknown[] and it would work just fine

wait, no, you still need the type for the keyof 🤔

and I think T extends U, U extends T won't work 🤔

[tiny arrow]

Yea I was thinking a third generic V for representing the value

Honestly might just go with unknown and not have typing on the key string comparator for ease of maintenance

Thank you for the help!