How to extend an interface declared in an external library d.ts?

You can easily extend the ‘knockout’ or any other TypeScript namespace.

Example: create knockout-extension.d.ts file

/// <reference path="<path-to-typings-dir>/knockout/index.d.ts" />

declare module 'knockout' {

  export interface CustomType {

    customField: string;

    customMethod(arg1: number, arg2: boolean): boolean;
  }

  namespace customNamespace {

    export interface AnotherCustomType {
      customField1: string;
      customField2: boolean;
    }
  }

  // NOTE: extending existing interface
  export interface KnockoutStatic {
    customMethod(): void;
  }
}

Note: ensure that this file is picked-up by the TypeScript compiler.

Use the newly defined types from the extended module.

// one way
import { CustomType } from 'knockout';

const foo: CustomType;

// second way
import * as kc from 'knockout';

const foo: kc.CustomType;
const bar: kc.customNamespace.AnotherCustomType;

For more info on modules and namespaces you can check TypeScript documentation on Modules and Namespaces and using them together.

Cheers!