A callback is a function passed to another function so it can run later, often after asynchronous work, an event, or a condition. Callbacks communicate a sequence such as “do something, then do something else,” but deeply nested callbacks can make asynchronous control flow harder to follow. section 8.2
Callbacks are an application of JavaScript’s first-class and higher-order functions: functions can travel as arguments or return values. Passed around as callbacks, they compose behaviorally by representing sequences of computational logic. section 4.0
Callback-based designs commonly appear in these forms:
- Event-driven operations, such as mouse events and HTTP requests, invoke supplied functions when the relevant event occurs. section 8.2
- Node.js-style asynchronous APIs often use error-first callbacks, handling an error before consuming returned data. section 8.2
- Promises provide a flatter alternative for sequencing asynchronous operations, while preserving the same broad “later, do something else” relationship. section 8.2
An error-first file-reading callback
fs.readFile('blocks.txt', (err, data) => { if (err) throw err; console.log(data); }); The callback receives either an error or the resulting data after the nonblocking operation completes. section 8.2
fs.readFile('blocks.txt', (err, data) => {
if (err) throw err;
console.log(data);
});
Use the file-reading listing to inspect the callback signature and see how asynchronous completion is handled without a synchronous blocking call. section 8.2
