Typescript use interface with strict values

Question

I've got a typescript/react project in which I am using @material components. There are definitions for those under @types/material__component. In the definitions there are often interfaces like (this one is from @types/material__menu/constants.d.ts):

export interface CornerBit extends MDCNumbers {
    BOTTOM: 1;
    CENTER: 2;
    RIGHT: 4;
    FLIP_RTL: 8;
}

Now if I wanna use CornerBit in my code I'd do:

import { CornerBit } from "@material/menu/"

@material/menu/index.js exports CornerBit from constants.js in which it is declared as:

const CornerBit = {
  BOTTOM: 1,
  CENTER: 2,
  RIGHT: 4,
  FLIP_RTL: 8,
};

Now when I import CornerBit like written above into my file and try to do:

const MyCornerBit = CornerBit;

I get:

TS2693: 'Corner' only refers to a type, but is being used as a value here.

So I guess I'm importing just the interface somehow, not the const itself. How do I use an interface like that (all properties have explicit values) though?

That just feels completely useless:

const MyCornerBit: CornerBit = {
    BOTTOM: 1,
    CENTER: 2,
    RIGHT: 4,
    FLIP_RTL: 8,
}

Thank you.

You’re trying to assign CornerBit to MyCornerBit and as the error says that’s not ok since it’s a type. If you want to say MyCornerBit is of type CornerBit then use : instead of =. It’s a bit unclear what you’re trying to achieve — Sami Kuhmonen
– Sami Kuhmonen, Commented Jul 31, 2018 at 9:47
Im trying to make MyCornerBit be an object with the values specified above. So that I can use MyCornerBit.BOTTOM, MyCornerBit.CENTER etc. — Schadenn
– Schadenn, Commented Jul 31, 2018 at 10:10

Dusan Radovanovic · Accepted Answer · 2018-07-31 09:50:46Z

1

You should just declare your const MyCornerBit as a CornerBit like this

const MyCornerBit: CornerBit;

There is no need to instantiate that const with matching properties

answered Jul 31, 2018 at 9:50

Dusan Radovanovic

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7 Comments

Schadenn Over a year ago

When I do const MyCornerBit: CornerBit; I get TS1155: 'const' declarations must be initialized

Dusan Radovanovic Over a year ago

declare with let instead const

Schadenn Over a year ago

Hmm and then I need to disable tslint, because it tells me to use const when MyCornerBit is not being reassigned. But is is actually working. So thank you. Still seems a little strange to me though.

Schadenn Over a year ago

Okay, sorry. It is NOT working actually. Getting no compile time errors or anything, but runtime is telling me "Cannot read BOTTOM from undefined"

Dusan Radovanovic Over a year ago

Does this interface CornerBit have an injectable decorator?

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