These are basic typings for Feathers.js based off of those in DefinitelyTyped for Express. This doesn't type feathers plugins, but does have some interfaces that can be used in them. WIP.

feathers.d.ts

// Type definitions for Feathers
// Project: http://feathersjs.com/
// Definitions by: Jack Guy <http://thatguyjackguy.com>
// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped

import { Application, Handler, ErrorRequestHandler } from 'express';
type HandlerArgument = Handler | Handler[];

export = Feathers;

declare function Feathers(): Feathers.FeathersApp;

declare namespace Feathers {

  // The app object returned from the feathers() constructor. Based on an Express app.
  export interface FeathersApp extends Application {
    // The version of Feathers from package.json
    version: string;

    methods: ['find', 'get', 'create', 'update', 'patch', 'remove'];
    services: any;
    providers: any[];

    // Since configure was removed from Express
    configure(Function): this;
    service(location: string): service.FeathersServiceInstance<any>;

    // Mixture of Express and Feathers - we have to duplicate these because there's no way
    // in TypeScript to "override" an extends. So we rewrite them all, express included.
    use(location: string, service: service.FeathersServiceConfig): this; // Feathers
    use(...handlers: HandlerArgument[]): this; // Express
    use(mountPoint: string | RegExp | (string | RegExp)[], ...handlers: HandlerArgument[]): this; // Express
    use(errorHandler: ErrorRequestHandler);

    setup(app: FeathersApp);
  }


  // Expose these to users for typings their hook creations
  export type Hooks = hooks.FeathersHooks;
  export type Hook<Resource> = hooks.FeathersHook<Resource>;
}

// Contains the typings for service-related features
declare namespace service {

    // The Events that can be subscribed to
    type FeathersEvent = 'create' | 'update' | 'patch' | 'remove';

    // Shared methods between a FeathersService and it's instance
    interface FeathersServiceMethods<Resource> {
        find?(params: FeathersParams, callback?: Function): PromiseLike<FeathersPage<Resource>>;
        get?(id: any, params: FeathersParams, callback?: Function): PromiseLike<Resource>;
        create?(data: Resource, params: FeathersParams, callback?: Function): PromiseLike<Resource>;
        update?(id: any, data: Resource, params: FeathersParams, callback?: Function): PromiseLike<Resource>;
        patch?(id: any, data: Resource, params: FeathersParams, callback?: Function): PromiseLike<Resource>;
        remove?(id: any, params: FeathersParams, callback?: Function): PromiseLike<Resource>;
    }

    // The returned / retrievable feathers.service (e.g. app.service('/users'))
    interface FeathersServiceInstance<Resource> extends FeathersServiceMethods<Resource> {
        before(hooks: hooks.FeathersHooks);
        after(hooks: hooks.FeathersHooks);
        on(event: FeathersEvent, callback: { (Resource): any });
    }

    // The interface that a new service object would implement
    interface FeathersService<Resource> extends FeathersServiceMethods<Resource>  {
        (FeathersServiceConfig): FeathersServiceInstance<Resource>;
        setup?(app: Feathers.FeathersApp, path?: string)
    }

    // Change pagination for a 'find' service call
    interface FeathersFindConfig {
        paginate?: {
            default?: number;
            max?: number;
        } | boolean;
    }

    // Customize the creation of a feathers service
    interface FeathersServiceConfig extends FeathersFindConfig {
        before?: hooks.FeathersHooks;
        after?: hooks.FeathersHooks;
    }

    // The parameters used by feathers services
    interface FeathersParams {
        query: any;
        data: any;
        result: any;
    }

    // Paginated resources returned by find
    interface FeathersPage<Resource> {
        total: number;
        limit: number;
        skip: number;
        data: Resource[];
    }

}

// Contains the typings for hook-related features
declare namespace hooks {

    // The typings for the "hook" parameter in hook functions
    interface FeathersHookInputBase {
        method: string;
        type: 'before' | 'after';
        callback: Function;
        params: any;
        data: any;
        app: Feathers.FeathersApp;
    }

    // We have an individual parameter typing for each hook type
    interface FeathersHookInputGet extends FeathersHookInputBase { id: any; }
    interface FeathersHookInputFind extends FeathersHookInputBase { }
    interface FeathersHookInputCreate extends FeathersHookInputBase { data: any; }
    interface FeathersHookInputUpdate extends FeathersHookInputBase { data: any; id: any; }
    interface FeathersHookInputPatch extends FeathersHookInputBase { data: any; id: any; }
    interface FeathersHookInputRemove extends FeathersHookInputBase { data: any; id: any; }
    type FeathersHookInputAll =
        FeathersHookInputGet | FeathersHookInputFind | FeathersHookInputCreate |
        FeathersHookInputUpdate | FeathersHookInputPatch | FeathersHookInputRemove;

    // The type of an actual hook function
    interface FeathersHook<Input> {
        (hook: Input): PromiseLike<any> | void
    }

    // Either an array of hooks or a single hook -
    // so we don't have to rewrite a super complicated type each time
    type FeatherHookArrayOrHook<Input> = Array<FeathersHook<Input>> | FeathersHook<Input>;
    interface FeathersHooks {
        all?: FeatherHookArrayOrHook<FeathersHookInputAll>;
        find?: FeatherHookArrayOrHook<FeathersHookInputFind>;
        get?: FeatherHookArrayOrHook<FeathersHookInputGet>;
        create?: FeatherHookArrayOrHook<FeathersHookInputCreate>;
        update?: FeatherHookArrayOrHook<FeathersHookInputUpdate>;
        patch?: FeatherHookArrayOrHook<FeathersHookInputPatch>;
        remove?: FeatherHookArrayOrHook<FeathersHookInputRemove>;
    }

}

Don't you have to wrap this in declare module "feathers" { ...}? I couldn't get this file to work as is. Had to tinker with it a bit to get completion and everything compiling. My final results are here.

Something to keep in mind too:

I'm guessing feathers was compiled by Babel? There's a known issue with using export default that causes feathers.default (in the compiled js) to be undefined. That means you have to use export = Feathers which then requires an import via import feathers = require('feathers'); or import * as feathers from feathers. That one took me a while to figure out.

Does anyone have an example on how this type definition can be used? I am trying to implement it and finding it to be somewhat difficult

@Chrisdf I copy it into an index.d.ts file in a custom_typings folder and then npm link it into @types/feathers. Works like a charm. :)