Class 1 ~ JavaScript for Beginners ~ SVA-DSI

JavaScript for Beginners

Class 1

What is JavaScript?

JavaScript is standardized by the "ECMAScript" specifications.
JavaScript is a client-side processing language. A browser reads the code and runs it directly.
JavaScript interfaces with HTML and CSS.
With JavaScript, you can write code once and use it everywhere. Remember, you want DRY code (Don't Repeat Yourself).
JavaScript lets you build dynamic webpages that respond to input from users.

JavaScript is a client-side language

Laptop and server connected via the internet

Photo credits: Andrew E. Larson and John Seb Barber cc

Practice

Let's write your first JavaScript program. Make a folder called sva. Inside, make a new page called index.html. Place this code inside.


<!DOCTYPE html>
<html>
    <head>
        <title>Test Page</title>
    </head>
    <body>
    <p>This is my awesome JavaScript code.</p>
        <script>
            alert('Hello World!');
        </script>
    </body>
</html>

Script Tags

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


<script>
    CODE GOES HERE
</script>

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.


<script>
    console.log('Hello World!');
    console.log('I am glad to meet you');
    console.log('I am fun');
</script>

Comments

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


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

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!');

Practice

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.

Variables

A variable is a place to store values

Variable Values

When you first create a variable, it does not have a value (it is null).
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.


<script>
    var numberOfKittens;
</script>

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


<script>
    var numberOfKittens = 5;
</script>

Using a Variable

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


<script>
    var numberOfKittens = 5;
    console.log (numberOfKittens);
</script>

Practice

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

Numbers

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


<script>
    var numberOfKittens = 5;
    var cutenessRating = 9.6;
</script>

The browser will automatically convert integers to floats if needed

Arithmetic Operators

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


<script>
    var numberOfKittens = 5;
    var numberOfPuppies = 4;
    var numberOfAnimals = numberOfKittens + numberOfPuppies;
</script>

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.

Practice

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

Strings

Variables can be strings, groups of characters. You put your string in quotes.


<script>
    var kittensName = 'Miss';
</script>

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


<script>
    console.log('I\'d like to use an apostrophe');
</script>

String Operators

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


<script>
    var kittensName = 'Miss';
    var fullName = kittensName + ' Fortune';
    console.log(fullName); //Outputs 'Miss Fortune'
</script>

String Operators

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


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

Practice

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

Functions are separable, reusable pieces of code.

Using Functions

First, declare the function.


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

Then, use it as many times as you want!


<script>
    turtleFact();
</script>

Arguments

Functions can accept input values, called arguments.


<script>
    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
</script>

Arguments

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


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

Practice

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.


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

Return will immediately end a function.

Practice

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

Resources

JavaScript Guide, from the Mozilla Developers Network.
Codecademy, with interactive JavaScript lessons to help you review.
Udacity, javascript basics course online.
Code School, javascript part 1 tutorial.
Dash, free online tool for JS/HTML/CSS integration from General Assembly.