Module augmentation in TypeScript

Adding to another module's types from your own code.

import type { IncomingMessage } from "node:http";

declare module "node:http" {
  interface IncomingMessage {
    requestId?: string;
    user?: { id: string; scopes: readonly string[] };
  }
}

declare module "node:querystring" {
  interface ParsedUrlQuery {
    typestarTrace?: string;
  }
}

export function tag(request: IncomingMessage, id: string): string {
  request.requestId = id;
  request.user = { id: "u-1", scopes: ["read"] as const };
  return `${request.requestId}:${request.user.scopes.length}`;
}

console.log(typeof tag);

How it works

1. Reopening the module merges your declarations into it.
2. The file must be a module, or the declaration is global.
3. This is how a plugin types the field it adds.

Keywords and builtins used here

• IncomingMessage
• as
• const
• declare
• export
• from
• function
• import
• interface
• readonly
• return
• string
• type

The run, in numbers

Lines

22

Characters to type

512

Tokens

117

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 64 seconds.

Type this snippet

Step 3 of 3 in Modules & declarations, step 7 of 8 in Async & modules.

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