Typescript: undefined calculated property after deserializing in JSON

Issue

I'm new to Typescript and I encountered a JSON deserializing problem.

Consider this class:

class Product {

    public id!: number;

    public get calculatedProperty() : string {
        return "Test";
    };

};

As you can see calculatedProperty is a runtime calculated property.

Also, consider that I deserialize a JSON string into my object in this way:

var jsonData = '{ "id": 2 }';
let deserialized = JSON.parse(jsonData) as Product;

The problem comes now:

• This call console.log(deserialized.id); returns correctly 1.
• This call console.log(deserialized.calculatedProperty); returns undefined!

I really don't understand way. It seems that as Product doesn't really create a Product object, because If I directly invoke the constructor, new Product, the calculated property exists.

What am I doing wrong with the JSON deserialization?

Thanks!

Solution

The JSON.parse method isn't really for converting json into a class rather than an object.

To solve your issue you could potentially convert the json into an object like this:

let deserializedObject = JSON.parse(jsonData) as Object;

and after that you could assign the object to a class like that:

let deserialized = Object.assign(new Product(), deserializedObject);

Note that I have not tested this yet, but it should work. Also this is fine for simple objects, but not for objects with complex hierarchy.

Look into class-transformer for more information. https://github.com/typestack/class-transformer

Answered By - jstnklnr