Typescript: declare variable with same type as another

Question

Is there any way to declare a variable with the type of another variable? For instance, I declare a class member with some type, and then later I want to declare another variable in a function of the same type. But I don't want to modify the original declaration, and don't want to duplicate it. It seems like you should be able to do something like:

class Foo {
    bar: {[key: string]: string[]};

    func() {
        const x: TypeOf<Foo.bar> = {};
        ....
    }
}

I've heard of something like this specifically for return types of functions, but I can't find it anymore...

You can write const x: Foo["bar"] = ... to determine the type. — ford04
– ford04, Commented Oct 11, 2019 at 13:54

Ivan · Accepted Answer · 2019-10-11 13:53:18Z

13

You can use typeof but in class you should get to property:

class Foo {
    bar: {[key: string]: string[]};

    func() {
        const x: typeof Foo.prototype.bar = {};
        // here x has type `{[key: string]: string[]}`
    }
}

And another example outside of class:

class A {
    b: string = ''
}

type test = typeof A.prototype.b // type is `string`

PlayGround

answered Oct 11, 2019 at 13:53

Ivan

2,5892 gold badges23 silver badges29 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Aluan Haddad Over a year ago

consider typeof this['b']

Tyler2P · Accepted Answer · 2022-07-26 14:15:14Z

1

Another way:

class Foo {
    bar: {[key: string]: string[]};

    func() {
        const x: Foo['bar'];
    }
}

It's called indexed access

More: https://www.typescriptlang.org/docs/handbook/2/indexed-access-types.html#handbook-content

edited Jul 26, 2022 at 14:15

Tyler2P

2,37030 gold badges26 silver badges34 bronze badges

answered Jul 25, 2022 at 10:02

Mateusz Daniluk

1252 silver badges10 bronze badges

Collectives™ on Stack Overflow

Typescript: declare variable with same type as another

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related