Callback

JavaScript contains functions such as setTimeout and requestAnimationFrame that are executed asynchronously by default. Often, after an asynchronous function is executed, other functions are supposed to be executed as a consequence. For this, callback functions are used as arguments. In addition, callback functions are often written as anonymous functions.

window.setTimeout(() => {
  console.log('Callback');
}, 5000);

Traditionally, the first parameter of callback functions contains the error state, i.e. the err parameter contains information about an error if something went wrong or is falsy if everything went as expected. This allows handling of the result of the execution of the function.

import { readFile } from 'fs';

readFile('something.txt', 'utf8', (err, data) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(data);
});