// Definitions by: Carlos Ballesteros Velasco <https://github.com/soywiz>
//                 Leon Yu <https://github.com/leonyu>
//                 BendingBender <https://github.com/BendingBender>
//                 Maple Miao <https://github.com/mapleeit>

/// <reference types="node" />
import * as stream from 'stream';
import * as http from 'http';

export = FormData;

declare class FormData extends stream.Readable {
  append(key: string, value: any, options?: FormData.AppendOptions | string): void;
  getHeaders(): FormData.Headers;
  submit(
    params: string | FormData.SubmitOptions,
    callback?: (error: Error | null, response: http.IncomingMessage) => void
  ): http.ClientRequest;
  getBuffer(): Buffer;
  getBoundary(): string;
  getLength(callback: (err: Error | null, length: number) => void): void;
  getLengthSync(): number;
  hasKnownLength(): boolean;
}

declare namespace FormData {
  interface Headers {
    [key: string]: any;
  }

  interface AppendOptions {
    header?: string | Headers;
    knownLength?: number;
    filename?: string;
    filepath?: string;
    contentType?: string;
  }

  interface SubmitOptions extends http.RequestOptions {
    protocol?: 'https:' | 'http:';
  }
}

// Definitions by: Carlos Ballesteros Velasco <https://github.com/soywiz>
//                 Leon Yu <https://github.com/leonyu>
//                 BendingBender <https://github.com/BendingBender>
//                 Maple Miao <https://github.com/mapleeit>

/// <reference types="node" />
import * as stream from 'stream';
import * as http from 'http';

export = FormData;

declare class FormData extends stream.Readable {
  append(key: string, value: any, options?: FormData.AppendOptions | string): void;
  getHeaders(): FormData.Headers;
  submit(
    params: string | FormData.SubmitOptions,
    callback?: (error: Error | undefined, response: http.IncomingMessage) => void
  ): http.ClientRequest;
  getBuffer(): Buffer;
  getBoundary(): string;
  getLength(callback: (err: Error | undefined, length: number) => void): void;
  getLengthSync(): number;
  hasKnownLength(): boolean;
}

declare namespace FormData {
  interface Headers {
    [key: string]: any;
  }

  interface AppendOptions {
    header?: string | Headers;
    knownLength?: number;
    filename?: string;
    filepath?: string;
    contentType?: string;
  }

  interface SubmitOptions extends http.RequestOptions {
    protocol?: 'https:' | 'http:';
  }
}

--- 
+++ 
@@ -14,11 +14,11 @@
   getHeaders(): FormData.Headers;
   submit(
     params: string | FormData.SubmitOptions,
-    callback?: (error: Error | undefined, response: http.IncomingMessage) => void
+    callback?: (error: Error | null, response: http.IncomingMessage) => void
   ): http.ClientRequest;
   getBuffer(): Buffer;
   getBoundary(): string;
-  getLength(callback: (err: Error | undefined, length: number) => void): void;
+  getLength(callback: (err: Error | null, length: number) => void): void;
   getLengthSync(): number;
   hasKnownLength(): boolean;
 }
Fix error in callback signatures

The functions actually give `null` as the error parameter, and not `undefined`.

Without this, `util.promisify` can't be typed correctly on these functions as it expects the callback to receive `Error | null`.