Typescript how do I dynamically use type of object property

Question

Say I have some code like this:

type Action = {
    type: string;
    data: /* need help here */;
};

// data is a string here
const action1: Action = {
    type: 'foo',
    data: 'bar'
};

// data is an object here
const action2: Action = {
    type: 'complex',
    data: {
        first: 'John',
        last: 'Doe'
    }
};

// data is not defined here
const action3: Action = {
    type: 'undef'
};

// and so on, the point is data can be anything

I want to have the type of data be whatever the type of data is on the object so that the previous code example will not give me any compiler errors. I don't want to use any because I want to have my editor's intellisense help with the type when I type. Is there a way in Typescript to do what I'm trying to do?

@caTS That's extremely close to what I want. I'm assuming the function used for initializing the objects is necessary? — Anthony Bias
– Anthony Bias, Commented Sep 23, 2022 at 15:25
Yes, otherwise you will have to use const act: Action<...> = { data: ... };, essentially duping the type you want. The function is there to infer the type for you. — k.tten
– k.tten, Commented Sep 23, 2022 at 15:27

k.tten · Accepted Answer · 2022-09-23 15:31:07Z

3

You can use a generic like this to represent that the type of data be potentially anything:

type Action<T> = {
    type: string;
    data?: T;
};

But when you define a variable, you would essentially have to duplicate the type, which is not useful. So that's why we need a function to infer the generic for us:

function action<T = undefined>(a: Action<T>): Action<T> {
    return a;
}

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answered Sep 23, 2022 at 15:31

k.tten

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