fix: Add typing for File#download() (#409)

Author: ThomasdenH

package.json

@@ -50,6 +50,7 @@
     "Stephen Sawchuk <sawchuk@gmail.com>",
     "Stephen Sawchuk <stephenplusplus@users.noreply.github.com>",
     "Tyler Johnson <mail@tyler-johnson.ca>",
+    "Thomas den Hollander <denhollander.thomas@gmail.com>",
     "Zach Bjornson <bjornson@stanford.edu>",
     "greenkeeper[bot] <greenkeeper[bot]@users.noreply.github.com>"
   ],

src/file.ts

@@ -122,6 +122,23 @@ export interface CopyOptions {
   userProject?: string;
 }
 
+/**
+ * @typedef {array} DownloadResponse
+ * @property [0] The contents of a File.
+ */
+export type DownloadResponse = [Buffer];
+
+/**
+ * @callback DownloadCallback
+ * @param err Request error, if any.
+ * @param contents The contents of a File.
+ */
+export type DownloadCallback = (err: Error|undefined, contents: Buffer) => void;
+
+export interface DownloadOptions extends CreateReadStreamOptions {
+  destination?: string;
+}
+
 interface CopyQuery {
   sourceGeneration?: number;
   rewriteToken?: string;
@@ -1306,15 +1323,6 @@ class File extends ServiceObject {
     (this.parent as ServiceObject).delete.call(this, options, callback);
   }
 
-  /**
-   * @typedef {array} DownloadResponse
-   * @property {object} [0] The contents of a File.
-   */
-  /**
-   * @callback DownloadCallback
-   * @param {?Error} err Request error, if any.
-   * @param {buffer} [contents] The contents of a File.
-   */
   /**
    * Convenience method to download a file into memory or to a local
    * destination.
@@ -1368,13 +1376,21 @@ class File extends ServiceObject {
    * region_tag:storage_download_file_requester_pays
    * Example of downloading a file where the requester pays:
    */
-  download(options?, callback?) {
-    if (is.fn(options)) {
-      callback = options;
+  download(options?: DownloadOptions): Promise<DownloadResponse>;
+  download(options: DownloadOptions, callback: DownloadCallback): void;
+  download(callback: DownloadCallback): void;
+  download(
+      optionsOrCallback?: DownloadOptions|DownloadCallback,
+      callback?: DownloadCallback): Promise<DownloadResponse>|void {
+    let options: DownloadOptions;
+    if (is.fn(optionsOrCallback)) {
+      callback = optionsOrCallback as DownloadCallback;
       options = {};
+    } else {
+      options = optionsOrCallback as DownloadOptions;
     }
 
-    callback = once(callback);
+    callback = once(callback as DownloadCallback);
 
     const destination = options.destination;
     delete options.destination;