Discovering the data type from its textual representation

Is it possible for TypeScript to automatically determine the generic argument of assertTypeof based on the value of expectedType?

I am looking for a way to use the function below without having to specify number multiple times.

playable example

type TypeLiteral = "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function"

// I know its bad to return generic, but dont know how to make without it
function assertTypeof<T>(expectedType: TypeLiteral, target: any): T {
    const actualType = typeof target
    if (actualType === expectedType) return target

    throw new Error(`Expected ${expectedType}, but got ${actualType}`)
}


const n1 = assertTypeof<number>('number', 1) // fine
const n2 = assertTypeof<number>('number', {}) // error
typescript

Answer №1

An effective way to represent string -> type mappings is by creating an interface and leveraging the indexed access type operator as the return type for the assertTypeof function:

interface TypeMapping {
    string: string,
    number: number,
    boolean: boolean,
    symbol: Symbol,
    undefined: undefined,
    object: object,
    function: Function
};

function assertTypeof<T extends keyof TypeMapping>(expectedType: T, target: any) : TypeMapping[T] {
    const actualType = typeof target;
    if (actualType === expectedType) return target;

    throw new Error(`Expected ${expectedType}, but received ${actualType}`);
}

const num1 = assertTypeof('number', 1); // works fine
const num2 = assertTypeof('number', {}); // throws a runtime error

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