Array.reduce doesn't work

[ilchenkoArtem]

Bug Report

🔎 Search Terms

Array.reduce
Array.reduce doesn't work

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about reduce

⏯ Playground Link

Playground link with relevant code

💻 Code

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🙁 Actual behavior

TS throws error "This expression is not callable."

🙂 Expected behavior

TS should let me iterate through array (I tell it the type is either array of IBaseOption OR array of ISelectOption (not an array of mixed types). In any case the variable is an array and those methods should be callable.

[jcalz]

The example code should be here as plain text, and phrases like "doesn't work" and "works" aren't really descriptive enough to do anybody much good. Could you make an edit?

Note that this seems to be a duplicate of or strongly related to #36390; that issue is closed as fixed, although the problem with reduce() remains; not sure if this one should stay open in its place or if there's a different canonical issue somewhere.

[RyanCavanaugh]

I think what you want is this (which has two edits)

TEST_LIST.map((testListItem) => {
  const options = testListItem.options;
  (options as IBaseOption[]).reduce<ValueType[]>((acc, option) => {
    if(option.parentValue) {
      acc.push(option.parentValue)
    }
   
    return acc;
  }, []);
})

There's no typesafe way for TS to collapse the different reduce definitions without knowing what reduce "actually does"; requiring a type assertion here is appropriate.

[Lafore17]

But one question reading comments above: considering options as IBaseOption[] | ISelectOption[] I expect to get type for option inside reduce like IBaseOption | ISelectOption and at the same time never[] for accum before setting type via generic. am I right?

[ialexryan]

I poked around in the files of #31023, which fixed .map() on unions of array types (closing #36390), and found this comment in the test suite:

https://github.com/microsoft/TypeScript/pull/31023/files#diff-7a118e355c5fce1fa4374a0e2b34732788f5695be1a6bf5b36bb71bcfa3b7fb9R28

which indicates that .reduce() not working was a known limitation. @weswigham – any chance you already have an issue tracking that somewhere, or is this ticket fine? I've also checked some other common array methods and created a new issue to track the ones that aren't working.

[github-actions]

This issue has been marked as 'Unactionable' and has seen no recent activity. It has been automatically closed for house-keeping purposes.