Array.reduce prevValue is type of array and not of initialValue type when we return undefined

[pkasarda]

Bug Report

Array.reduce is not using the proper type when working with prevValue, it should have type of initialValue but it doesn't

🔎 Search Terms

reduce, array, initialValue, currentValue

🕗 Version & Regression Information

all versions also nightly

⏯ Playground Link

Playground link with relevant code

undefined

nullish

💻 Code

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🙁 Actual behavior

it shows prevValue as number

🙂 Expected behavior

it should show prevValue as initialValue type

[fatcerberus]

initialValue is the initial value of previousValue, not currentValue. currentValue is always an element of the source array, so the type looks correct to me.

[fatcerberus]

const arr: number[] = [0,1,2,3,4,5];

type ArrObj = {
  subRows: ArrObj[];
}

const arrObj: ArrObj[] = [
  { subRows: [] as ArrObj[] },
  { subRows: [] as ArrObj[] },
]

const result = arr.reduce((prevValue, currentValue) => {
   console.log(currentValue);
   return null as any as ArrObj;
}, arrObj[0]);

prints

0
1
2
3
4
5

number is the correct type for currentValue here.

[pkasarda]

sorry everyone... I was in rush :( issue is/was with prevValue not with initialValue... and steps are HERE not sure but when object can be undefined then it's talking about number from somewhere...

but fixed even when I'm using it like this

[andrewbranch]

Through the edits and updates, I have no idea what the issue is here or if you still have one, but if you’re unsatisfied with reduce inference, it’s probably a duplicate of #46438

[fatcerberus]

@andrewbranch Yeah, I have no idea either - OP now says prevValue is the wrong type (number instead of ArrObj), but the playground links show prevValue correctly being inferred as ArrObj, so...

It seems like the actual problem is OP's mistake - initialValue is an ArrObj and so that's what the callback is expected to return, but it might in fact return something nullish, causing a type error. It seems suspiciously like a gross misunderstanding of what reduce actually does.

[pkasarda]

@andrewbranch yes, it looks like a duplicate of #46438

@fatcerberus forget to remove comment from playground, I updated the link in my original comment...

[fatcerberus]

@pkasarda If this is a duplicate of #46438 then your issue was the type of the result, not the prevValue/currentValue (which seem to be inferred correctly in all of your examples). Even more to the point, the inferred types don’t matter, since there was an error (“no overload matches this call”) on the callback that you didn’t mention. So it wasn’t at all clear what you were even trying to do.