JS - JavaScript

What is JavaScript?

  • Standardized by the ECMAScript specifications.
  • Client-side processing language. A browser reads the code and runs it directly.
  • Interfaces with HTML and CSS.
  • Dynamic webpages that respond to input from users

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>

Separating Instructions

After each individual statement, you must add a semicolon.

      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 and computers will ignore.

      /*I can wrap long comments
      with multiple lines
      like this*/
      console.log('Hello World!'); //Or mark short comments like this

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 your message.
  • Create a new file called mycode.js. Move your code to this file and link it to your page.


A variable is a place to store values

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 information, like words, numbers, and collections of data.
  • The value of a variable can change over time.

Naming Variables

  • The variable name is case-sensitive.
  • A new variable needs to have a unique name.
  • Variable names need to start with a letter, $, or _.
  • Variable names can only be made of letters, numbers, $, or _.

Declaring a Variable

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

      var numberOfKittens = 5;

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;
      console.log (numberOfKittens);

Let's Develop It

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


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

      var numberOfKittens = 5;
      var cutenessRating = 9.6;

The browser will automatically convert integers to floats if needed

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.

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 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');

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'

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!


Functions are separable, reusable pieces of code.

Using Functions

First, declare the function.

      function turtleFact() {
        console.log('A turtle\'s lower shell is called a plastron.');

Then, use it as many times as you want!



Functions can accept input values, called arguments.

      function callKitten (kittenName){
        console.log('Come here, ' + kittenName + '!');
      callKitten ('Fluffy'); //outputs 'Come here, Fluffy!'

      function addNumbers(a, b) {
        console.log(a + b);
      addNumbers(5,7); //outputs 12
      addNumbers(9,12); //outputs 21


You can also pass variables into functions. These variables do not need to have the same name as the function arguments.

      function addOne(inputNumber){
        var newNumber = inputNumber + 1;
        console.log('<p>You now have ' + newNumber);
      //Declare variables
      var numberOfKittens = 5;
      var numberOfPuppies = 4;
      //Use them in functions

Let's Develop It

Turn the code you wrote to output someone's full name into a function, then use it.

Returning Values

You can have a function give you back a value, to use later.

      function square(num) {
        return num * num;
      console.log(square(4));   // outputs '16'.
      var squareOfFive = square(5); // will make squareOfFive equal 25.

Return will immediately end a function.

Let's Develop It

Add a return statement to your name function. Use that function to set the value of a variable.