Question

119

Typescript: How to map object to type?

rated 0 times [ 125] [ 6] / answers: 1 / hits: 16949 / 7 Years ago, tue, january 9, 2018, 12:00:00

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?

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jayla

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answered 7 Years ago andreasn · Accepted Answer

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)