Beginning Javascript

Class 2


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Some "rules"

  • We are here for you!
  • Every question is important
  • Help each other
  • Have fun


Sometimes you want to go through a piece of code multiple times


  • Showing a timer count down
  • Displaying the results of a search
  • Adding images to a slideshow

The while loop

The while loop tells JS to repeat statements while a condition is true:

      while (expression) {
        // statements to repeat

      var x = 0;
      while (x < 5) {
Review: '++' means increment by 1!

What happens if we forget x++;?

The loop will never end!!

The for loop

The for loop is a safer way of looping

          for (initialize; condition; update) {
            // statements to repeat

          for (var i = 0; i < 5; i++) {
Less danger of an infinite loop. All conditions are at the top!


An array is a data-type that holds an ordered list of values, of any type:

          var arrayName = [element0, element1, ...];

var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green', 'Blue', 'Indigo', 'Violet'];
var favoriteNumbers = [16, 27, 88];
var luckyThings = ['Rainbows', 7, 'Horseshoes'];
The length property reports the size of the array:


Arrays -- returning values

You can access items with "bracket notation".

The number inside the brackets is called an "index"

          var arrayItem = arrayName[indexNum];
Arrays in JavaScript are "zero-indexed", which means we start counting from zero.

      var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green', 'Blue', 'Indigo', 'Violet'];
      var firstColor = rainbowColors[0];
      var lastColor = rainbowColors[6];

Arrays -- updating values

You can also use bracket notation to change the item in an array:

        var awesomeAnimals = ['Corgis', 'Otters', 'Octopi'];
        awesomeAnimals[0] = 'Bunnies';
Or to add to an array:

          awesomeAnimals[4] = 'Corgis';
You can also use the push method:


Loops and Arrays

Use a for loop to easily look at each item in an array:

  var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green', 'Blue', 'Indigo', 'Violet'];
  for (var i = 0; i < rainbowColors.length; i++) {


It's natural to think in collections, sets, groups.
Remember the "Danger" of infinite loops with while?
"for each" is better, and so to is "for in", but...
YOU are ultimately responsible for the loop, and bugs happen
Instead if your using ECMA 5 (IE9+) try ".forEach"

  var rainbowColors = ['Red', 'Orange', 'Yellow', 'Green', 'Blue', 'Indigo', 'Violet'];
  //No infinite loop, no indexing, can be any function.
  rainbowColors.forEach(function (color) {

Let's Develop It

  • Add a new link to the exercise from last week
  • Add an onclick to the link for a function called favoriteThings()
  • Create a new function called favoriteThings() in the javascript file
  • In the function, create an array and loop through the results
  • Post the results in an alert "My favorite things are XX, YY, ZZ'
  • Bonus -- add an 'and' in the sentence before the last item


Objects are a data type that let us store a collection of properties and methods.

          var objectName = { 
            propertyName: propertyValue,
            propertyName: propertyValue,

      var charlie = {
        age: 8,
        name: "Charlie Brown",
        likes: ["baseball", "The little red-haired girl"],
        pet: "Snoopy"

Objects -- returning values

Access values of "properties" using "dot notation":

      var charlie = {
        age: 8,
        name: "Charlie Brown",
        likes: ["baseball", "The little red-haired girl"],
        pet: "Snoopy"

          var pet =;

Objects -- returning values

Or using "bracket notation" (like arrays):

          var name = charlie['name'];
Non-existent properties will return undefined:

          var gender = charlie.gender

Objects -- changing values

Use dot or bracket notation with the assignment operator to change objects.

Change existing properties:

 = "Chuck";
Or add new properties:

          charlie.gender = "male";
You can also delete properties:

          delete charlie.gender;

Arrays of Objects

Arrays can hold objects too!

          var peanuts = [
            {name: "Charlie Brown", 
             pet: "Snoopy"},
            {name: "Linus van Pelt",
             pet: "Blue Blanket"}
That means we can use a for loop!

  for (var i = 0; i < peanuts.length; i++) {
    var peanut = peanuts[i];
    console.log( + ' has a pet named ' + + '.');

Objects in functions

You can pass an object into a function as a parameter

        var peanut ={
          name: "Charlie Brown", 
          pet: "Snoopy"

  function describeCharacter(character){
    console.log( + ' has a pet named ' + + '.');


Let's Develop It

  • Add another link that calls the function myFriends() onclick
  • Add a new function to the javascript myFriends
  • In the function, create an array of friends objects, with their names and hair colors
  • Use a for loop to go through each friend and describe them
  • Alert the results
  • Bonus -- make a separate functions that describe the friends


  • "Document Object Model"
  • A way to interact with the HTML elements on a webpage
  • Chrome and Firefox -- Right click --> Inspect Element

DOM Interaction

On every webpage, the document object gives us ways of accessing and changing the DOM.

Every DOM "node" has properties. They are connected like a family tree.

Parent (parentNode), children (childNodes, firstChild), siblings (prevSibling, nextSibling)

  var bodyNode = document.body; // <body>
  var htmlNode = document.body.parentNode; // <html>
  for (var i = 0; i < document.body.childNodes.length; i++) {
    var childNode = document.body.childNodes[i];
    //could be <p>, <h1>, etc.
    //any html element

DOM Interaction: ID

Finding every element on the page by siblings and children is time consuming!

The document object also provides methods for finding DOM nodes without going one by one

Find element by id

<img id="mainpicture" src="">

var img = document.getElementById('mainpicture');            
Best practice of ECMA v1 (1997)

DOM Interaction: Easier

ID are supposed to be unique
The utillity of giving everything a unique "id" runs out quick
Other mehods were added to fix this problem in HTML5/ES5 (2009)

  <img id="mainpicture" src="">
  <span class="javascript-tips">Learn with GDI</span>

  var listItems = document.getElementsByTagName('img');
  var classyItems = document.getElementsByClassName('javascript-tips');
Better still, but more "uselike" than "usefull"

DOM Interaction: Easiest

There is an easier way

      var elements = document.querySelectorAll('span.important');
Using query / style selectors
Same pattern as jQuery (2006)

      var elements = $('span.important');
Very concice, and uses knowlege of style slectors.


  • Methods are functions that are associated with an object
  • The affect or return a value for a specific object
  • Used with dot notation
Previously seen example:

            var img = document.getElementById('mainpicture');

DOM Nodes -- Attributes

We can use node methods to set and retrieve attributes


 var img = document.getElementById('mainpicture');
 img.setAttribute('src', '');

 var img = document.getElementById('mainpicture');
 img.setAttribute('class', 'picture-class');


Each DOM node has an innerHTML property:

You can set innerHTML yourself to change the contents of the node:

document.body.innerHTML = '<p>I changed the whole page!</p>';
You can also just add to the innerHTML instead of replace everything:

document.body.innerHTML += "...just adding this bit at the end of the page.";

DOM Modifying

The document object can create new nodes:


var newImg = document.createElement('img');
newImg.src = '';

var newParagraph = document.createElement('p');
var paragraphText = document.createTextNode('New Paragraph!');

Let's Develop It

  • Put it all together
  • Modify your existing three functions to add new elements to the screen instead of fire an alert
  • Keep in mind how to find an element, how to append an element, and how to change the inner html of an element
  • There are lots of possible solutions! Be creative!