One of the main feature of PocketBase is that it can be used as a framework which enables you to write your own custom app business logic in Go or JavaScript and still have a portable backend at the end.

Choose Extend with Go if you are already familiar with the language or have the time to learn it. As the primary PocketBase language, the Go APIs are better documented and you'll be able to integrate with any 3rd party Go library since you'll have more control over the application flow. The only drawback is that the Go APIs are slightly more verbose and it may require some time to get used to, especially if this is your first time working with Go.

Choose Extend with JavaScript if you don't intend to write too much custom code and want a quick way to explore the PocketBase capabilities. The embedded JavaScript engine is a pluggable wrapper around the existing Go APIs, so most of the time the slight performance penalty will be negliable because it'll invoke the Go functions under the hood.
As a bonus, because the JS VM mirrors the Go APIs, you would be able migrate gradually without much code changes from JS -> Go at later stage in case you hit a bottleneck or want more control over the execution flow.

With both Go and JavaScript, you can:

• Register custom routes:
  Go

  JavaScript

  app.OnBeforeServe().Add(func(e *core.ServeEvent) error { e.Router.GET("/hello", func(c echo.Context) error { return c.String(http.StatusOK, "Hello world!") }, apis.ActivityLogger(app), apis.RequireAdminAuth()) return nil })

  routerAdd("GET", "/hello", (c) => { return c.string(200, "Hello world!") }, $apis.activityLogger($app), $apis.requireAdminAuth())
• Bind to event hooks and intercept responses:
  Go

  JavaScript

  app.OnRecordBeforeCreateRequest("posts").Add(func(e *core.RecordCreateEvent) error { requestInfo := apis.RequestInfo(e.HttpContext) // if not an admin, overwrite the newly submitted "posts" record status to pending if requestInfo.Admin == nil { e.Record.Set("status", "pending") } return nil })

  onRecordBeforeCreateRequest((e) => { let requestInfo = $apis.requestInfo(e.httpContext) // if not an admin, overwrite the newly submitted "posts" record status to pending if (!requestInfo.admin) { e.record.set("status", "pending") } }, "posts")
• Register custom console commands:
  Go

  JavaScript

  app.RootCmd.AddCommand(&cobra.Command{ Use: "hello", Run: func(cmd *cobra.Command, args []string) { print("Hello world!") }, })

  $app.rootCmd.addCommand(new Command({ use: "hello", run: (cmd, args) => { console.log("Hello world!") }, }))
• and many more...

For further info, please check the related Extend with Go or Extend with JavaScript guides.