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Introduction to JavaScript

Class 2

Let's Look Back

Woman looking backward as she dances

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Code Search

In this code, spot the comment, variables, and operators.

var billPreTip = 10;
var tipPercent = 0.15; // Can be changed

var billTip = billPreTip * tipPercent;
var receipt = 'Meal: ' + billPreTip + ' Tip: ' + billTip +
' Total: ' + (billPreTip + billTip);



Functions are separable, reusable pieces of code. Pile of legos

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Declaring Functions

To declare (create) a function, you can give it a name, then include all the code for the function inside curly brackets {}

function parrotFacts() {
  console.log('Some parrot species can live for over 80 years');
  console.log('Kakapos are a critically endangered flightless parrot');

Functions can have multiple lines.

Using Functions

To invoke (use) a function, you type its name, followed by parenthesis ()


We'll talk about what can go inside those parenthesis in a minute! For now, leave them empty.

What's going on here?

A function is a group of code you can reuse many times. Whenever you invoke a function by using it's name, you tell the browser to run the code inside the function.

You must declare a function before you can use it.

Let's Develop It

Write a function that outputs a sentence. Then invoke that function later in your code.


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(num){
  var newNumber = num + 1;
  console.log('You now have ' + newNumber);

// Declare variables
var numberOfKittens = 5;
var numberOfPuppies = 4;

// Use them in functions

Let's Develop It

Write a simple program to combine a first name and a last name inside a function. Then update the function to accept a first and last name as arguments.

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); // squareOfFive equals '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.

Variable Scope

The scope of a variable determines where it's value is accessible throughout your program.

Footprints being washed away

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Global Scope

A variable declared outside of a function has a global scope and can be accessed anywhere, even inside of functions.

var awesomeGroup = 'Girl Develop It'; // Global scope

function whatIsAwesome() {
  console.log(awesomeGroup + ' is pretty awesome.'); // Will work


Local Scope

A variable declared inside a function has a local scope and can only be accessed within that function.

function whatIsAwesome() {
  var awesomeGroup = 'Girl Develop It';              // Local scope
  console.log(awesomeGroup + ' is pretty awesome.'); // Will work


console.log(awesomeGroup + ' is pretty awesome.'); // Won't work

Boolean Variables

Kitten with light switch

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Boolean Variables

Boolean variables represent the logical values true and false

var catsAreBest = true;
var dogsRule = false;

Boolean Variables

Some values are considered falsy and will evaluate to false in a Boolean context.

// the following variables will evaluate as false

var myName = '';
var numOfKids = 0;
var isMarried;     // remember a variable with no value is undefined

null and NaN will also evaluate as false.

Everything else evaluates as true.

Control Flow

Forked path

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The if statement

Use if statements to decide which lines of code to execute, based on a condition.

if (condition) {
  // statements to execute
var age = 30;

if (age > 18) {
  console.log('You are an adult');

Comparison Operators

Example Name Result
a == b Equal TRUE if a is equal to b (can be different types).
a === b Identical TRUE if a is equal to b, and the same type.
a != b Not equal TRUE if a is not equal to b (can be different types).
a !== b Not identical TRUE if a is not equal to b, or they are not the same type.
a < b Less than TRUE if a is strictly less than b.
a > b Greater than TRUE if a is strictly greater than b.
a <= b Less than or equal to TRUE if a is less than or equal to b.
a >= b Greater than or equal to TRUE if a is greater than or equal to b.

Watch out!

Don't mix up = and == and ===

Caution Tape

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Let's Develop It

Make a variable called "temperature". Write some code that tells you to put on a coat if it is below 50 degrees.

Even more control flow

Sign post with multiple signs

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The if/else statement

Use else to provide an alternate set of instructions.

var age = 30;

if (age >= 16) {
  console.log('Yay, you can drive!');
} else {
  console.log('Sorry, you have ' + (16 - age) +
  ' years until you can drive.');

The if/else statement

If you have multiple conditions, you can use else if.

var age = 30;

if (age >= 35) {
  console.log('You can vote AND run for President!');
} else if (age >= 30) {
  console.log('You can vote AND run for the Senate!');
} else if (age >= 18) {
  console.log('You can vote!');
} else {
  console.log('You can\'t vote, but you can write your representatives.');

Let's Develop It

Modify your "wear a coat" code for these conditions:

  1. If it's less than 50 degrees, wear a coat.
  2. If it's less than 30 degrees, wear a coat and a hat.
  3. If it's less than 0 degrees, stay inside.
  4. Otherwise, wear whatever you want.

Logical Operators

Example Name Result
a && b And TRUE if both a and b are TRUE.
a || b Or TRUE if either a or b is TRUE.
!a Not TRUE if a is not TRUE.

Using logical operators

You can use these operators to combine conditions.

var age = 30;
var yearsAsCitizen = 30;

if (age >=30 && yearsAsCitizen > 9) {
  console.log('You can run for the Senate!');
} else {
  console.log('You are not eligible to run for the Senate');

Let's Develop It

Add a logical operator to your "what to wear" program.

You did it!

Happy cat

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