Summary - Callbacks

Build a file-read simulation using an error-first callback.

Summary - Callbacks

In this group you learned the foundations of callbacks. A callback is a function you pass as an argument to another function. Asynchronous callbacks are invoked later by the event loop. In Node.js, the error-first convention makes the first parameter the error. Nested callbacks create callback hell; you fix it by splitting into named functions.

Now you will write two tasks that bring it all together: a file-read simulation using an error-first callback, and a sequence of timers split into named functions.

Exercise Write a readFile simulation

Define a function named readFile. It should take two parameters: filename and callback. Inside the function, use setTimeout to check for an error after 100 ms. If filename is 'missing.txt', call callback with new Error(...). Otherwise, call callback with null and a content string.

Show example solution

function readFile(filename, callback) {
  setTimeout(() => {
    if (filename === 'missing.txt') {
      callback(new Error('File missing'));
    } else {
      callback(null, 'File content');
    }
  }, 100);
}

Exercise A sequential flow with named functions

Define two named functions: loadData and showData. loadData should take a callback parameter and use setTimeout to call callback with a data string after 100 ms. showData should take a data parameter and log it with console.log. Finally, call loadData(showData).

Show example solution

function loadData(callback) {
  setTimeout(() => {
    callback('Data loaded');
  }, 100);
}
function showData(data) {
  console.log(data);
}
loadData(showData);

Test yourself

If there is no error in an error-first callback, what is the first parameter?
- undefined
- null
- false
- An empty string
Show answer

null — Correct! When there is no error, the first parameter is null; the result is the second parameter.
Match the concepts

callback

Called-back function
event loop

Runs async callback
err parameter

First argument
named function

Callback hell fix

Next lesson: What is a promise?

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