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Callbacks Review

Lesson Objectives

  • Define and use a callback function
  • Execute a delayed JavaScript function using callbacks to timing functions
  • Handle an event with a callback function
  • Identify callback function signatures used with various Array method higher-order functions


A callback function is a function reference passed as an argument to another function.

function callFunctionPassedIn (fnPassedIn) {

Event Listeners

Document Ready


function initDoc () {


  console.log('the document was clicked', event.timeStamp, 'milliseconds since page load')

We Do: $ jQuery Hijink$

//first run this...
jQuery('body').html('') // $ is an alias for jQuery! they refer to the same thing

//then run these
  console.log(this, ' was clicked!')
  $(this).css('background', 'red')

Array Methods: Higher-Order Functions

Callback Function Signature & Composition

function diagram

Array methods like .map, .sort, and .reduce each take callback functions that have unique function signature.



function (currentValue, index, array)

this function acts upon each item that map is iterating over and must have a return in order for .map to return a changed array.

[1,2,3,4,5].map(function (currentValue, index, array) {
  console.log(currentValue, index, array)
  return currentValue * 2



function (currentValue, index, array)

this function also acts upon each item that filter is iterating over and returns a truthy or falsey value that determines whether or not the item will be included in the array returned by .filter.

[1,2,3,4,5].filter(function (currentValue, index, array) {
  console.log(currentValue, index, array)
  return currentValue % 2 === 1



function (a, b) //comparison function which compares a & b

const numbers = [11, 42, 39, 14, 25]


function compare (a, b) {
  // return a - b
  // how is the return above working?
  // see the breakdown below from MDN

  // a should come before b
  if (a is less than b by some ordering criterion) {
    return -1 // => a, b

  // b should come before a
  if (a is greater than b, or comes after, by the ordering criterion) {
    return 1 // => b, a

  // a must be equal to b
  return 0 //remain in place



function (accumulator, currentValue, index, array)

const numbers = [11, 42, 39, 14, 25]

numbers.reduce((total, current) => total + current)
// we're not providing an accumulator here, so reduce will take the first item 
// from the array reduce is called on, and use that value at the 0th index
// as the starting value of the accumulator
// as a consequence, currentValue will be the item at index 1 in the array
// that .reduce() is being called on
// same as above but with a named function
function add (firstNum, secondNum) {
  return firstNum + secondNum

const numbers = [11, 42, 39, 14, 25]

const sum = numbers.reduce(add)
Object Histogram

Here, an object {} will function as our accumulator. It's going to help us count the number of times each name occurs in the array. Each key in the object will be the name in array, and the corresponding value will be how many times that names occurs.

//from MDN
var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

var countedNames = names.reduce(function (allNames, name) { 
  if (name in allNames) {
  else {
    allNames[name] = 1;
  return allNames;
}, {});
// countedNames is:
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

Timing Functions


Timeouts run functions after a delay, specified in milliseconds.

function alertPopUp () {
  alert("Pop Up")

const numberOfMilliseconds = 1000 // 1 second

setTimeout(alertPopUp, numberOfMilliseconds)


setInterval does a couple things: it calls the function passed in every so often, depending on the number of milliseconds specified. It works just like setTimeout, but the action happens repeatedly. Since the function is going run indefinitely, we will want to have a means of stopping that function from running. We have a function that can do this, clearInterval, but we also need a way of identify which recurring process we want to end. Luckily, setInterval returns a whole number which function as that recurring action's id.

// anonymous callback function
const timerId = setInterval(function () {
  console.log('One second has just passed.')
}, 1000)

//wait a bit and then run...