在 React/Typescript 中将元素作为类型传递给 function

Question

I am trying to create a generic collection for a React project in Typescript. I have:

class List<T>
{
    constructor(data: any){...}
}

which will create a list of a type. I want to cast it to something by calling it with something like:

let carList: List<Car> = ...
carList.cast(Car)

if assuming Car is a class.

How do I pass in a class type as a parameter to map it? I have this so far:

public cast(Type: T)
{
    this.list = this.list.map(x => x as Type)
}

which does not work as the compiler complains about 'Type refers to a value but is being used as a type'. Assume this.list is created when the List is initialized.

Answer 1

Type does not exists in JS runtime, so you can not pass it as function parameter.

Usually to cast your List to List in the runtime you have to check is it possible. So you have to check all list content, and it could be expensive from the performance point of view

type Guard<T> = (item: any) => item is T;

// https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards

class List<T> {

    constructor(
        public data: T[]
    ) { }

    static getUserGuard<T>(itemGuard: Guard<T>): Guard<List<T>> {
        return (list: List<any>): list is List<T> => list.data.every(itemGuard)
    }
}

// and list usage

const lists: List<any>[] = [
    new List<number>([42]),
    new List<string>(['foo']),
]

const isNumber = (v: any): v is number => typeof v === 'number';
const isString = (v: any): v is string => typeof v === 'string';

// some code with random lists
const doSomething = (lists: List<any>[]) => {
    for (const list of lists) {
        if (List.getUserGuard(isNumber)(list)) {
            // list is List<number> there
        } else if (List.getUserGuard(isString)(list)) {
            // list is List<string> there
        } else {
            // list has some other type there
        }
    }
}

Also you can check it my classes but not by the content

class List<T> {
    constructor(
        public data: T[]
    ) { }
}

class NumbersList extends List<number> {

}

class StringList extends List<string> {

}

// some code with random lists
const doSomething = (lists: List<any>[]) => {
    for (const list of lists) {
        if (list instanceof  NumbersList) {
            // list is List<number> there
        } else if (list instanceof  StringList) {
            // list is List<string> there
        } else {
            // list has some other type there
        }
    }
}