Typescript extend third-party declaration files

Question

How can I extend third-party declaration files?
for example, I want to extend Context from @types/koa and add an extra field(resource) to it.
I tried this:

// global.d.ts
declare namespace koa {
    interface Context {
        resource: any;
    }
}

But it doesn't work:

error TS2339: Property 'resource' does not exist on type 'Context'.

Update

a simplified version of my code which produces this error:

import {Context} from 'koa';
import User from './Models/User';
class Controller {
   async list(ctx: Context) {
        ctx.resources = await User.findAndCountAll();
        ctx.body = ctx.resources.rows;
        ctx.set('X-Total-Count', ctx.resources.count.toString());
        ctx.status = 200;
    }
}

typescript v2.4

// tsconfig.json
{
  "compilerOptions": {
    "target": "es6",
    "module": "commonjs",
    "moduleResolution": "node",
    "noImplicitAny": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  },
  "exclude": [
    "node_modules"
  ]
}

Please post a [verifiable](https://stackoverflow.com/help/mcve) example. Note that "it doesn't work" is explicitly listed as an insufficient description of a problem. — jcalz, Sep 29 '17 at 16:37
Remove `declare`. The namespace has already been declared by the third party library, you are just extending it. And you have to say `export interface ` inside your namespace. [`Docs Here`](https://www.typescriptlang.org/docs/handbook/declaration-merging.html#merging-namespaces) — mhodges, Sep 29 '17 at 16:38
@mhodges `A 'declare' modifier is required for a top level declaration in a .d.ts file.` — Saman Mohamadi, Sep 29 '17 at 16:44
If @Saravana's answer doesn't fix your problem, please include the code that's producing the error. — jcalz, Sep 29 '17 at 17:53

score 54 · Accepted Answer · answered Sep 29 '17 at 17:41

54

You have to use module augmentation as described here:

import { Context } from "koa";

declare module "koa" {
    interface Context {
        resource: any;
    }
}

answered Sep 29 '17 at 17:41

Saravana

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Did you try adding this to a separate `*.d.ts` file? – Saravana Sep 29 '17 at 18:16
Yes, It's currently located in global.d.ts, but I have tested with another separate file and nothing changed. – Saman Mohamadi Sep 29 '17 at 18:23
I tried with the exact same code you are using and it works for me. Can you post something that can be reproduced. – Saravana Sep 30 '17 at 03:48
4

How do you do this without the "import" so an ambient file remains ambient and doesn't module-up and stop working? – Jefferson Mar 18 '21 at 19:35
I expanded the FastifyInstance with this trick. Many thanks – Kasir Barati Dec 29 '22 at 15:39

Typescript extend third-party declaration files

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