Mastering JavaScript Functional Programming - Second Edition

Callbacks, promises, and continuations

Probably the most used example of functions passed as first-class objects has to do with callbacks and promises. In Node, reading a file is accomplished asynchronously with something like the following code:

const fs = require("fs");fs.readFile("someFile.txt", (err, data) => {  if (err) {    console.error(err); // or throw an error, or otherwise handle the problem  } else {    console.log(data.toString()); // do something with the data  }});

The readFile() function requires a callback—in this example an anonymous function—that will get called when the file reading operation is finished. 

A better way is using promises; read more at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise ...