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A function you hand to another function to be called later.
A function passed as an argument to another function, to be invoked when some event happens or some work completes. Common in event handlers (onClick), array methods (map, filter), and async APIs.
A higher-order function pattern for inversion of control. The callee owns when and how the callback runs. In Node.js the convention is `(err, result) => void` as the last argument; this 'callback hell' nesting motivated the move to promises and async/await.