Why does TypeScript infer the 'never' type when reducing an Array with concat?

typescript functional-programming reduce reducers strictnullchecks
millsp picture millsp · Jan 9, 2019 · Viewed 12k times · Source

Code speaks better than language, so:

['a', 'b', 'c'].reduce((accumulator, value) => accumulator.concat(value), []);

The code is very silly and returns a copied Array...

TS complains on concat's argument: TS2345: Argument of type 'string' is not assignable to parameter of type 'ConcatArray'.

Answer

Matt H picture Matt H · Jan 9, 2019

I believe this is because the type for [] is inferred to be never[], which is the type for an array that MUST be empty. You can use a type cast to address this:

['a', 'b', 'c'].reduce((accumulator, value) => accumulator.concat(value), [] as string[]);

Normally this wouldn't be much of a problem since TypeScript does a decent job at figuring out a better type to assign to an empty array based on what you do with it. However, since your example is 'silly' as you put it, TypeScript isn't able to make any inferences and leaves the type as never[].

Related questions

TypeScript 3.0 error on `unknown` usage

Here, I test TypeScript3.0 unkown type. https://blogs.msdn.microsoft.com/typescript/2018/07/12/announcing-typescript-3-0-rc/#the-unknown-type TypeScript 3.0 introduces a new type called unknown that does exactly that. Much like any, any value is assignable to unknown; however, unlike any, you …

Read More
Typescript - Higher order function types

I'm getting really excited about TypeScript. How do you set the type of a function parameter? function twoMoreThanYou(calculateANumber: Function):number { return calculateANumber(4) + 2; } function double(n:number):number { return n*2; } console.log("TWO MORE", twoMoreThanYou(double)) How can I type …

Read More
How to convert a string to number in TypeScript?

Given a string representation of a number, how can I convert it to number type in TypeScript? var numberString: string = "1234"; var numberValue: number = /* what should I do with `numberString`? */;

Read More