Introduction to JavaScript

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Class 1


Girl Develop It is here to provide affordable and accessible programs to learn software through mentorship and hands-on instruction.

Some "rules"

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


Tell us about yourself.

  • Who are you?
  • What do you hope to get out of the class?
  • What's your favorite ice cream flavor?

What is JavaScript?

Java logo with a no symbol

JavaScript is not Java

JavaScript is the language of the web

Laptop and server connected via the internet

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JavaScript works with HTML & CSS


Photo credit: Gillicious cc

JavaScript lets you reuse code

Desert landscape

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What is JavaScript?

  • Created by Brendan Eich as "LiveScript" in 1995, which got renamed to "JavaScript"
  • Standardized by the ECMAScript specifications. This class covers ES5 (standardized in 2009)
  • A client-side processing language. A browser reads the code and runs it directly.
  • Interfaces with HTML & CSS.
  • Lets you build dynamic webpages that respond to input from users.

What can JavaScript do?

Image lightboxes

What can JavaScript do?

Fully featured web applications

What can JavaScript do?

Keep track of users with Cookies or storing data with local storage.

What can JavaScript do?

Interactive elements like tabs, sliders, etc

What can JavaScript do?

Drawing & animation

What can JavaScript do?


Script Tags

You can mix JavaScript and HTML. The script tag tells your browser the stuff inside is code, not content.


JavaScript Files

Just like CSS, you can split a long block of JavaScript into its own file.

<script src="path/to/file.js"></script>

Let's Develop It

  • Make a folder called gdi.
  • Inside, make a new page called index.html.
  • Write this code inside.
  • <!DOCTYPE html>
        <title>Test Page</title>
        <p>This is my awesome JavaScript code.</p>
          alert('Hello World!');
          console.log('Secret message');

Computers need simple, clear instructions

Confused robot

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Thinking like a programmer

Computers are great at processing. They are bad at understanding.

When you write a program, you must break down every step into simple pieces.

Example: Draw a Square

  1. Find a whiteboard and a dry erase marker.
  2. Uncap the dry erase maker.
  3. Hold the marker in your hand.
  4. Place the marker against the whiteboard.
  5. Move your hand 1 foot to the right.
  6. Stop.
  7. Move your hand 1 foot down...

Example: Make a Sandwich

Source: Harvard Students Making Sandwich: CS 50 Algorithm Intro

Basically, computers can be hard to work with!

Toddler using old word processor

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So what's up with JavaScript?


Photo credit: Adam Foster cc

How does JavaScript work?

  1. You visit a website with JavaScript code on it.
  2. Your browser (e.g., Chrome) reads the code line-by-line.
  3. The browser runs each line of code as it reads it.
  4. Based on these instructions, the browser performs calculations and changes the HTML and CSS on the page.
  5. If the browser finds code it doesn't understand, it stops running and creates an error message.


You can see what's going on in the console.

Let's Develop It

  • Open the console.
  • In Chrome, use the keyboard shortcut:
    • Mac: Command + Option + J
    • Windows: Control + Shift + J
  • Open your practice page.
  • Do you see anything in the console?
  • Try typing in 2 + 2 and hitting enter.


Each instruction in JS is a "statement", like:

console.log('Hello World!');
console.log('I am glad to meet you');
console.log('I am fuzzy');


You can leave comments in your code—notes that people can read but computers will ignore.

I can make long comments
with multiple lines here

console.log('Hello World!'); // Or make short comments here

Getting results onto your screen

Open a popup box.

alert('Hello World!');

Display a message in your console.

console.log('Hello World!');

Add something to the page.

document.write('Hello World!');

Let's Develop It

  • Open index.html.
  • Add a comment to the code.
  • Try different ways of printing a message.
  • Create a new file called mycode.js.
  • Move your code to this file and link it to your page.


Just like 'x' in algebra, a variable is a named container for a value that can change.

Empty glass

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Declaring a Variable

To declare (create) a variable, just type the word var and the variable name.

var numberOfKittens;

It is a good idea to give your variable a starting value. This is called initializing the variable.

var numberOfKittens = 5;

Variable Values

  • When you first create a variable, it does not have a value (it is undefined).
  • You can set a value for a variable.
  • Variables can hold different types of data.
  • The value of a variable can change over time.

Naming Variables

  • The variable name is case-sensitive.
  • A new variable should have a unique name.
  • Variable names need to start with a letter, $, or _.
  • Avoid reserved words.
  • Choose clarity and meaning for humans to read later.

Using a Variable

Once you have created a variable, you can use it in your code. Just type the name of the variable.

var numberOfKittens = 5;

Let's Develop It

In your JavaScript file, create a variable and give it a valid name and value. Then, display the value.

Data Types

  • string string of characters
    var userName = 'Jane Lane';
  • number integer or floating point
    var myAge = 30;
  • boolean true or false
    var catsAreBest = true;
  • undefined value that hasn't been defined
    var favoriteThings;
  • null an explicitly empty value
    var goodPickupLines = null;


Cats in a basket

Photo credit: WJ van den Eijkhof cc


Variables can be numbers, either integers or floats (decimals).

var numberOfKittens = 5;
var cutenessRating = 9.6;

JavaScript automatically converts integers to floats

NaN = Not-A-Number

Arithmetic Operators

Once you have numbers, you can do math with them!

var numberOfKittens = 5;
var numberOfPuppies = 4;
var numberOfAnimals = numberOfKittens + numberOfPuppies;

Arithmetic Operators

Example Name Result
-a Negation Opposite of a.
a + b Addition Sum of a and b.
a - b Subtraction Difference of a and b.
a * b Multiplication Product of a and b.
a / b Division Quotient of a and b.
a % b Modulus Remainder of a divided by b.

Let's Develop It

Create two variables and try some arithmetic operators. Don't forget to display your results!


Variables can be strings (groups of characters). You put your string in single or double quotes.

var kittensName = 'Fluffy';

If you want to use a quote in your string, you'll need to escape it with a backslash.

console.log('I\'d like to use an apostrophe');

Playing with Strings

Cat playing with string

Photo credit: Mike Lawson cc

String Operators

You can put strings together with a +, the concatenation operator.

var kittensName = 'Fluffy ';
var fullName = kittensName + 'McDougle';
console.log(fullName); // Outputs 'Fluffy McDougle'

String Operators

You can also use += to add things to the end of a string.

var kittensName = 'Admiral ';
kittensName += 'Snuggles';
console.log(kittensName); // Outputs 'Admiral Snuggles'


Cat jumping on another cat.

Photo credit: Matt cc

Let's Develop It

Create two variables, a first name and a last name, and then put them together to make a full name. Don't forget to display your results!

Combining strings and numbers

You can use concatenation to mix strings and numbers. When you do this, JavaScript will treat the number like a string.

var numberOfFruit = 6;
var typeOfFruit = 'bananas';
var allTheFruit = 'I have ' + numberOfFruit + ' ' + typeOfFruit + '!';

Let's Develop It

Create a program to calculate the tip at a restaurant. It should:

  • Have variables for the bill pre-tip and the tip percentage.
  • Calculate the total bill.
  • Output a sentence like "Your total bill, with tip, is $14.75".
  • Bonus: Use toFixed() to round the bill total to 2 decimals.

You did it!

People celebrating

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