In this code the type of array is already defined but Typescript fires an error for the array push, saying that:

Argument of type 'string | number' is not assignable to parameter of type 'never'. Type 'string' is not assignable to type 'never'.

type Characters = {
    letters: string[];
    numbers: number[];
}

const characters :Characters = {
    letters:  ['a', 'b',' c' ],
    numbers: [1,2,3],
}

type AddCharacter = <K extends keyof Characters, V extends Characters[K][number]>
(characterType:K, character:V) => void;

const addCharacter  :AddCharacter = (characterType, character) =>{
   characters[characterType].push(character) //Type 'string' is not assignable to type 'never'
}

addCharacter('letters', 'd') //ok
addCharacter('numbers', 4) //ok
addCharacter('letters', 4) //error as expected beacause 4 is not a string
addCharacter('numbers', 'd') //error as expected beacause d is not a number
addCharacter('words', 'test' )  //error as expected because 'words' is not a key of 'Characters'

I cannot figure out why it showing error since the type seem to be correct (got perfect type checking calling addCharacter). How can I get rid of the error and getting it to work nicely with typescript? Thanks for any help! Playground

CodePudding user response：

If you insert the following line into the body of addCharacter,

let x = characters[characterType][1]

You'll see that Typescript infers the type of x to be string | number, which implies that characters[characterType] is string[] | number[].

That would explain why, if you inspect the type of the called push method, the items parameter has type never[],

(method) Array<T>.push(items: never[]): number

because T is string[] | number[], not just string[] or just number[]. This causes the type of items to be never[], which in turn results in the error message you see.

You can fix the issue by changing addCharacter to:

const addCharacter:AddCharacter = <K extends keyof Characters, V extends Characters[K][number]>(characterType: K, character: V) => {
    (characters[characterType] as V[]).push(character)
}

This forces the type of characters[characterType] to be the dynamically inferred type of V based on the type of K. I don't know why Typescript doesn't do this automatically. Perhaps a Typescript guru can explain. Or perhaps we should file a Typescript Issue.