Functions, closures and this

In JavaScript, functions are values, and React Native code passes them everywhere: onPress handlers, callbacks for array methods and hooks. Learn function declarations and arrow functions, plus default and rest parameters. Then learn closures: a function keeps access to the variables from where it was created.

Closures explain the most common React bug, the stale closure. That is when a handler or effect still sees a value from an earlier render. Learn how this works too.

A regular function gets this from how it is called, while an arrow function takes it from the code around it. Modern React code uses functions and hooks rather than classes, so this matters less than it once did.