Extending typescript interface

Question

When extending the Express.Request interface in TypeScript I ran into this problem that I want to use an external library definition, but I can't import the external library as it results in error ->

Error:(4, 28) TS1147: Import declarations in an internal module cannot reference an external module.

Edit: It is a .d.ts file

/// <reference path="../typings/express/express.d.ts" />

declare module Express {
    import bunyan = require('bunyan'); <-- results in error
    export interface Request {
        _id: string; <-- this works
        log: bunyan.Logger; <-- Here I want to define that it is bunyan.Logger instance;
    }
}

Trying to reference the bunyan.d.ts (https://github.com/borisyankov/DefinitelyTyped/blob/master/bunyan/bunyan.d.ts) Also results in a problem, as the bunyan module is exported as string

declare module "bunyan" {
...
}

As such trying to use it from reference results in not found.

/// <reference path="../typings/express/express.d.ts" />
/// <reference path="../typings/bunyan/bunyan.d.ts" />

declare module Express {
    export interface Request {
        _id: string;
        log: bunyan.Logger; <- Error:(8, 18) TS2304: Cannot find name 'bunyan'.
    }
}

tl;dr; How to extend interface definition with external module definitions.

@Tarh Error:(4, 1) TS1046: A 'declare' modifier is required for a top level declaration in a .d.ts file. — DeadAlready
OK. The require misled me, I believed it was a .ts file. — Paleo
A suggestion: if a file .d.ts exists for the external module bunyan, you can reference it with /// <reference ... and then use it to type Request.log. — Paleo

David Sherret David Sherret · Accepted Answer · 2015-04-08T14:03:24

I don't think you can add to an existing interface when a require is necessary, but you can extend the existing interface using the extends keyword.

Move your import statement outside your module, export your module, and extend the existing interface:

import bunyan = require('bunyan');
import express = require('express');

export declare module ExtendedExpress {
    export interface Request extends express.Express.Request {
        _id: string;
        log: bunyan.Logger;
    }
}

Then you have to import this module where you want to use it.

Extending typescript interface

2 Answers