Typescript: How to map object to type?

Question

Assume that I have an object containing some data. I want to build a generic mapper (only a function respectively - I don't want to instantiate a new class all the time) for all types to use like this: this.responseMapper.map<CommentDTO>(data);

It should simply take all properties from the given type and map the data to it. What I tried so far:

public map<T>(values: any): T {
    const instance = new T();

    return Object.keys(instance).reduce((acc, key) => {
        acc[key] = values[key];
        return acc;
    }, {}) as T;
}

new T(); will throw an error: 'T' only refers to a type, but is being used as a value here.

What's the correct way to do this?

Answer 1

You need to pass the type constructor to the method. Typescript erases generics at runtime to T is not known at runtime. Also I would tighten values a bti to only allow memebers of T to be passed in. This can be done with Partial<T>

public map<T>(values: Partial<T>, ctor: new () => T): T {
    const instance = new ctor();

    return Object.keys(instance).reduce((acc, key) => {
        acc[key] = values[key];
        return acc;
    }, {}) as T;
 }

Usage:

class Data {
    x: number = 0; // If we don't initialize the function will not work as keys will not return x
}

mapper.map({ x: 0 }, Data)