The Callback Pattern is an important one to master when crafting reliable JavaScript because it is heavily used, in the language proper and in many third‐party libraries, such as JQuery. This chapter illustrates how to create reliable callback functions when interacting with built‐in JavaScript functions and third‐party libraries. The chapter introduces the Callback Pattern via a series of unit tests. The unit tests illustrate how to create callback functions, and how to write functions that accept callbacks. The chapter exposes common problems and mistakes that occur when implementing the Callback Pattern and ways to avoid and fix them. The first step in achieving the testing goals is to create some sort of callback function that can keep a record of each time it is executed, including the arguments that were provided when it was executed. The “callback arrow” is an extreme case of the use of anonymous functions as callbacks.