Typescript union type array `a[] | b[]` is not `reduce`able, but `map`able

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I'm trying to reduce over a union of array-types: foo[] | bar[]. With map it works as expected and I get an element of foo|bar in the lamda, but it does not work with reduce. I get the error, that 'the expression is not callable, because the signatures are not compatible'.

Maybe this is related, but that was in TS 3.6 days, and I'm using 4.5. Is there a (neat) way around this?

I have a Typescript Playground here

const m = [] as Array<number | string>;
m.reduce((it) => it); // ok
m.map((it) => it); // ok

const m2 = [] as Array<number> | Array<string>;
m2.reduce((it) => it);
m2.reduce((it) => it as any);  // not even with 'any'-lambda
// This expression is not callable.
// Each member of the union type '<OMITTED> ...' has signatures, but none of those signatures are compatible with each other.
m2.map((it) => it); // why does `map` work then ?

// same with tuples
const m3 = [1, 1] as [number, number] | [string];
m3.map((it) => it);
m3.reduce((it) => it); // same error
union-types typescript type-inference lambda
1 Answers

In addition to jcalz comment I found the following.

Typescript Definitions

The function signature for map is

export function map<T, U>(iterable: Iterable<T>, mapper: (value: T, index: number, iterable: Iterable<T>) => U): U[];

It has a different return type U then it's input T.

One of the function signatures for reduce is

export function reduce<T>(
   iterable: Iterable<T>,
    reducer: (previousValue: T, currentValue: T, currentIndex: number, iterable: Iterable<T>) => T,
initialValue?: T
): T;

It has the same return type as it's input (T). The trouble comes when assigning the type back unto itself. Typescript doesn't (and shouldn't) know which type to use.

One solution is to type check before using reduce, with a type guard like so,

const isNumberArray = (arr: unknown[]): arr is number[] => {
  for (let i = 0; i < arr.length; i++) {
      const element = arr[i]

      if (typeof element !== 'number') {
          return false
      }
  }

  return true
}

Playground

user_43172010
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