You move to api-calls.js . The previous developer used old-school callback functions. The code is indented ten times to the right—the dreaded "Pyramid of Doom." It’s unreadable. Furthermore, the keyword this is broken inside the callbacks, referring to the global window instead of the Drone object, causing the drone navigation system to fail.
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Rewind to June 2015. ECMAScript 2015 (ES6) drops like a bombshell. After 6 years of stagnation, JavaScript suddenly gets classes, arrow functions, template literals, destructuring, promises, modules, let / const — features that made it look like a real language. You move to api-calls