Poor type inference for reduce

TypeScript Version: 3.0.0-dev.20180705

Code

function toStrings(arr: ReadonlyArray<object>): string[] {
	return arr.reduce((acc, obj) => {
		acc.push(obj.toString());
		return acc;
	}, [] as string[]);
}

Expected behavior:

No error.

Actual behavior:

src/a.ts:2:2 - error TS2322: Type 'object' is not assignable to type 'string[]'.
  Property 'length' is missing in type '{}'.

2  return arr.reduce((acc, obj) => {
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3  	acc.push(obj.toString());
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
4  	return acc;
  ~~~~~~~~~~~~~
5  }, [] as string[]);
  ~~~~~~~~~~~~~~~~~~~~

src/a.ts:3:7 - error TS2339: Property 'push' does not exist on type 'object'.

3  	acc.push(obj.toString());

No error if arr is ReadonlyArray<number> or some other non-object type.
No error if I explicitly specify arr.reduce<string[]>.
No error if I remove the first two overloads to reduce, which are non-generic.

I think this goes here:

const things: Things[] = [{num:1}, {num:5}, {num: 2}];
this.things.slice(0, index).reduce((prevVal, curVal): number => {
      return prevVal.num + curVal.num;
    });

Error:
Argument of type '(prevVal: Things, curVal: Things) => number' is not assignable to parameter of type '(previousValue: Things, currentValue: Things, currentIndex: number, array: Things[]) => Things'.

Expected result is no error. We should be allowed to reduce an object to a single value that isn't the same object without using an initializer.

William Neely (@willyboy)

William Neely (@willyboy) It's indicating a bug in your code.

You're not passing an intial value into .reduce(), so JavaScript expects both prevVal and curVal to be Things. But you're returning a number from the callback, which goes into prevVal.

TypeScript says that you have => number but it should be => Things. This is a legit, helpful error message.

Check out this snippet for comparison.

I can add something to this.

I have an array with zeroes and ones created from a comparison (the example I'm posting here is a stupid version of it, just so the array type is (0 | 1)[]).

The reduce result is a positive number, but TS infers it will be 0 | 1.

const arr = [0, 1, 0, 0, 1, 1, 1, 1, 1, 1].map(v => v === 0 ? 0 : 1)
const sum = arr.reduce((p, c) => p + c, 0)

I have a similar issue:

const recordArray: Record<string, any>[] = [
    { 'a': 1, 'b': 'B', 'c': [1, 2, 3] },
    { 'd': 4, 'e': 'E', 'f': [4, 5, 6] },
    { 'g': 7, 'h': 'H', 'i': [7, 8, 9] }
];

const reducedArray = recordArray.reduce(
    (map, record) => map,
    new Map<string, string>()
);

In this code reducedArray and the map parameter inside the reduce function are being inferred of type Record<string, any> instead of Map<string, string>. This also happens if the accumulator is an array or a Set and probably with many other types as well, but it works with simple types like string or number.

If recordArray is a Record<K, T> where T isn't any (even if it's unknown) the reduce seems to correctly infer its return type and the type of the accumulator.

The example above is a pretty stupid example but I think it shows the issue pretty well. My real code looks more like the following:

const reducedArray = recordArray.reduce(
    (map, record) => condition
        ? map.set(...)
        : map,
    new Map<...>()
);

Edit: The example below is actually a simple and useful example. It converts a Record<string, any>[] into a Map<number, any>:

const record: Record<string, any>[] = ...

const map = record.reduce(
    (acc, item, index) => acc.set(index, item.property),
    new Map<number, any>()
);

In this example the expected types for map and acc is Map<number, any> but they are inferred as Record<string, any> instead.

Again the interesting part is that it seems to work as long as T isn't any in Record<K, T> and you can argue that the fix is to don't use any but still, it's a bug

Vulcagon (@Vulcagon)

Vulcagon (@Vulcagon) You can work around this problem by typing the accumulator in the callback:

const reducedArray = recordArray.reduce(
    (map: Map<string, string>, record) => map,
    new Map<string, string>()
);

But I agree that this should be considered a bug and fixed! When an initial value is passed into reduce, the accumulator in the callback (and thus the return type of reduce) should not be inferred from the source array.

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Poor type inference for `reduce` #25454

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Poor type inference for reduce #25454

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willyboy commented on Mar 28, 2021

lolmaus commented on Mar 28, 2021

russoedu commented on Jun 8, 2022

Vulcagon commented on Mar 22, 2025

lolmaus commented on Mar 24, 2025

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Poor type inference for `reduce` #25454