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Introduction to JavaScript

Class 3


Lego stormtrooper inside key ring

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While loops

while will repeat the same code over and over until some condition is met.

var bottlesOfBeer = 99;

while (bottlesOfBeer > 0) {
  console.log(bottlesOfBeer + ' bottles of beer on the wall');
  bottlesOfBeer = bottlesOfBeer - 1;

Infinite Loops

Make sure something changes in the loop, or your loop will go on forever...

Spiral that goes on toward infinity

Photo credit: Samuel John cc

For loops

for loops are very similar, but you declare a counter in the statement.

// will count 1 to 10
for (var i = 1; i <= 10; i++) {

Loops and logic

You can add other statements or logical operators inside the loops.

// Count from 1 to 100

for (var i = 1; i <= 100; i++) {
  if (i % 3 === 0) {
    // Says 'Fizz' after multiples of three
    console.log(' Fizz');
  } else if (i % 5 === 0) {
    // Says 'Buzz' after multiples of five
    console.log(' Buzz');
  } else {


To exit a loop, use the break statement.

// Count from 100 to 200
for (var i = 100; i <= 200; i++) {
  console.log('Testing ' + i);

  //Stop at the first multiple of 7
  if (i % 7 == 0) {
    console.log('Found it! ' + i);

Let's Develop It

Write a loop that gives you the 9's times table,
from 9 x 1 = 9 to 9 x 12 = 108.

Finish early? Try using a loop inside a loop to write all the times tables, from 1 to 12.


Group of kittens

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Arrays are ordered lists of values.

var arrayName = [value0, value1];

You can put different types of data into an array.

var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green',
  'Blue', 'Indigo', 'Violet'];

var lotteryNumbers = [33, 72, 64, 18, 17, 85];

var myFavoriteThings = ['Broccoli', 1024, 'Sherlock'];

Array Length

The length property tells you how many things are in an array.

var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green',
'Blue', 'Indigo', 'Violet'];


Using Arrays

You can access items with bracket notation by using the position of the item you want.

var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green',
'Blue', 'Indigo', 'Violet'];

var firstColor = rainbowColors[0];
var lastColor  = rainbowColors[6];

JS arrays are zero-indexed, so counting starts at 0.

Changing arrays

You can use bracket notation to change an item in an array

var myFavoriteThings = ['Broccoli', 1024, 'Sherlock'];

myFavoriteThings[0] = 'Asparagus';

Expanding arrays

Arrays do not have a fixed length. You can use push to add something to an array.

var myFavoriteThings = ['Broccoli', 1024, 'Sherlock'];


Let's Develop It

Create an array of your favorite foods.
Print a few values onto your screen.

Arrays + loops = BFF

Couple holding hands

Photo credit: John cc

Iterating through arrays

Use a for loop to easily work with each item in an array.

var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green',
  'Blue', 'Indigo', 'Violet'];

for (var i = 0; i < rainbowColors.length; i++) {

Let's Develop It

Use a for loop to print a list of all your favorite foods.


Collection of matches

Photo credit: Jirí Volejník cc


Objects let us store a collection of properties.

var objectName = {
  propertyName: propertyValue,
  propertyName: propertyValue
var user = {
  hometown: 'Atlanta, GA',
  hair: 'Auburn',
  likes: ['knitting', 'code'],
  birthday: {month: 10, day: 17}

Accessing Objects

You can retrieve values using dot notation

var user = {
  hometown: 'Atlanta, GA',
  hair: 'Auburn'

var usersHometown = user.hometown;

Or using bracket notation (like arrays)

var usersHair = user['hair'];

Changing Objects

You can use dot or bracket notation to change properties

var user = {
  hometown: 'Atlanta, GA',
  hair: 'Auburn'
}; = 'blue';

Add new properties

user.married = true;

Or delete them

delete user.married;

Arrays of Objects

Because arrays can hold any data type, they can also hold objects.

var users = [
  {name: 'Jolene', age: 21},
  {name: 'Alexa',  age: 18}

for (var i = 0; i < users.length; i++) {
  var user = users[i];
  console.log( + ' is ' + user.age + ' years old.');


Just like other data types, objects can be passed into functions:

var jolene = {
  age: 21,
  hairColor: 'Auburn',
  likes: ['pizza', 'tacos'],
  birthday: {month: 3, day: 14, year: 1995}

function describeUser(user) {
  console.log('You are ' + user.age + ' years old with '
  + user.hairColor + ' hair.');


Let's Develop It

Create an object to hold information on your favorite recipe. It should have properties for:

  • recipeTitle (a string)
  • servings (a number)
  • ingredients (an array of strings)
  • directions (a string)

Try displaying some information about your recipe.

Bonus: Create a loop to list all the ingredients.

Object methods

Objects can also hold functions.

var jolene = {
  age: 21,
  hairColor: 'Auburn',
  talk: function() {
  eat: function(food) {
    console.log('Yum, I love ' + food);

Call object methods using dot notation:;'pizza');

Let's Develop It

Go back to your recipe object. Add a function called letsCook that says "I'm hungry! Let's cook..." with the name of your recipe title.

Call your new method.

You did it!


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