How to inherit class variable type from constructor?

Question

I am writing a validator class, something like

class Validator {
  private path: string;
  private data: unknown;

  constructor(path: string, data: string) {
    this.data = data;
    this.path = path;

  }

  public isString() { /* ... */ }
}

Right now my data is of type unknown, but I'd like type to be inherited from constructor i.e.

const validator = new Validator("HomePage", 123); // data in class should be inherited as number

with functions I usually did something like

function<T>(path: string, data: T) {  }

But I am unable to figure out how to do it with classes. In particular inheriting from constructor

Do you mean something like this?

VLAZ
– VLAZ

2020-07-21 09:42:52 +00:00
Commented Jul 21, 2020 at 9:42 — VLAZ
– VLAZ, Commented Jul 21, 2020 at 9:42

Roberto Zvjerković · Accepted Answer · 2020-07-21 09:44:19Z

2

It's really similar as with functions:

class Validator<T> {
  constructor(private path: string, private data: T) {

  }
}

const validator = new Validator<string>('', '');

Also that's not called inheritance, it's just a generic parameter.

You also shouldn't have private path string, constructor parameters already do that for you.

answered Jul 21, 2020 at 9:44

Roberto Zvjerković

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1 Comment

VLAZ Over a year ago

Just to add to this - the generic value <string> can be omitted and TS will still try to guess it. For example new Validator("", "hello") will automatically infer T to be string. Passing an explicit generic argument is still fine but might not be needed in some cases, If you want something like new Validator<"hello" | "world">("", "hello") then you do need to be explicit, however.

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