Pfthb

I have two simple types of data:

type IData = 
 percent: string;
 exchange: string;
;

type IRow = 
 price: number;
 exchange: string;
;

I'm not following why typescript throws errors here if I use them both:

const sumSame = (
 assets: IData | IRow,
 sumBy: string,
 sumWhat: string
) => 
 const mapWithUniqCoins = assets.reduce((accMap, el: IData ;

But it doesn't throw errors when I combine two types into one shape:

type Shape = IData | IRow;

Why it happened?
https://codesandbox.io/s/16vzvqqv3 (Check Errors.ts and NoErrors.ts)

When assets are declared as a union of arrays

assets: IData | IRow

assets.reduce() has different declarations in each array type: for IData array, reduce() takes a callback that receives IData element, for IRow array, reduce() takes a callback that receives IRow element.

The error message - "Cannot invoke an expression whose type lacks a call signature" - is basically telling that the compiles is not smart enough to recognize that it could be possible to call reduce() for the union type, even when it receives appropriate callback type.

There is open issue for this:

This is currently by design because we don't synthesize an
intersectional call signature when getting the members of a union type
-- only call signatures which are identical appear on the unioned type