How can I declare a readonly property in a declarations file?

Question

The following doesn't seem to compile:

declare namespace ns {
    interface Test {
        readonly x: number;
    }
}

with:

Cannot find name 'readonly'.
Property or signature expected.

nor does this:

declare namespace ns {
    interface Test {
        const x: number;
    }
}

with:

Property or signature expected.

The playground uses TS 1.8. The modifier `readonly` will be a feature of TS 2.0. — Paleo, Jun 28 '16 at 10:06
@Paleo Can you include a source for this, and write it as an answer? — Zev Spitz, Jun 28 '16 at 10:08

score 3 · Accepted Answer · answered Jun 28 '16 at 10:41

Your example is compiled without error by TypeScript 2.0:

declare namespace ns {
    interface Test {
        readonly x: number;
    }
}

The current release is 1.8. In order to test with the next version, on a local installation: 1/ install the nightly build of TypeScript with npm install typescript@next, then 2/ execute the compiler: ./node_modules/.bin/tsc your-file.ts.

score 1 · Answer 2 · edited May 23 '17 at 12:25

1

You can't assign a value to a property in an interface in TypeScript. So how would you set a readonly variable if you are not allowed to initially set it.

You can take a look at the answer to this question here how you could solve this using a module instead:

module MyModule {
    export const myReadOnlyProperty = 1;
}

MyModule.myReadOnlyProperty= '2'; // Throws an error.

Update

It seems you have to wait for TypeScript 2.0 for this, which will have readonly properties then:

https://github.com/Microsoft/TypeScript/pull/6532

edited May 23 '17 at 12:25

Community

1
1

answered Jun 28 '16 at 09:19

malifa

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3

_You can't assign a value to a property in an interface in TypeScript._ True, but I can assign a value to the property of an object that implements the interface, and in this case I want to prevent that. _So how would you set a readonly variable..._, this is for an ambient declaration, which means I am not setting the property's value; that is being done by the library. – Zev Spitz Jun 28 '16 at 09:35
@ZevSpitz true, sorry i kinda responded to fast without thinking enough ;) This seems to be a similar question, but the answer won't help you out much i guess: http://stackoverflow.com/questions/14796751/how-can-i-add-a-readonly-attribute-to-a-declarations-file – malifa Jun 28 '16 at 10:22

How can I declare a readonly property in a declarations file?

2 Answers2

Update