Poor type inference for reduce

[ghost]

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.

[willyboy]

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.

[lolmaus]

@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.

[russoedu]

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)