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JavaScript Callbacks Lab


This lab provides an opportunity to practice working with callbacks.

This lab is not a deliverable.

Note: This can be a pair programming activity or done independently.



  1. Copy each of the exercises to a HTML,CSS,JS Repl
  2. Code away!


// Exercise 1

// A fellow student shows you this code.  When he runs it, he expects it to
// wait three seconds, then write "Ding!" to the console.  Instead, it writes
// "Ding!" immediately.  Find the bug and fix it.

function writeDing() {

var dingHandle = setTimeout(writeDing(), 3000);
// Exercise 2

// Javascript arrays have a built-in sort method that can take
// a callback to tell it how to compare the things you want to sort.

// Research the array sort method.

// Write the sorting callback as a named function declaration

// Write the callback function to provide to the sort method so that
// the following code sorts the words from shortest to longest.

var words = ['short', 'medium', 'delicious', 'nice', 'lengthy'];

var sortedWords = words.sort(/* pass in a named callback here */);

// Check that logging sortedWords outputs
// ["nice", "short", "medium", "lengthy", "delicious"]
// Exercise 3
// Change Exercise 2 so that:
//   1. The words sort longest to shortest
//   2. Use an anonymous inline function

var longWordsFirst = words.sort(/* write an anonymous inline function here */);

// Check that logging longWordsFirst outputs
// ["delicious", "lengthy", "medium", "short", "nice"]
// Exercise 4

// Let's pretend that we want to build a sandwich, but adding each
// ingredient is very cpu intensive, so we want to write each function
// that adds a certain ingredient to be written as an asynchronous
// function that, of course, accepts a callback that it will call after
// the ingredient has been added.

// The following are the completed functions to build a sandwich with:

function getBread(sandwich, cb) {
  setTimeout(function() {
  }, 1000);

function addMayo(sandwich, cb) {
  setTimeout(function() {
  }, 900);

function addTurkey(sandwich, cb) {
  setTimeout(function() {
  }, 800);

function addCheese(sandwich, cb) {
  setTimeout(function() {
  }, 700);

// Variable to hold the sandwich's ingredients
var mySand = [];

// Assignment: Call the above functions so that logging mySand
// after "cheese" has been added
// produces these ingredients in the following order:
// ["bread", "mayo", "turkey", "cheese"]
// Hint: Remember those nested callbacks when calling the step1, step2...


Write function named countdown that accepts as an arg the starting number of seconds and console.logs the count down to zero one second apart from each other.

For example:


console.logs something like the following:

Count: 3
Count: 2
Count: 1
Count: 0

Solution Code

This links to a CodePen containing the solution code