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Sungkyunkwan University

Copyright 2000-2012 Networking LaboratoryCopyright 2000-2018 Networking Laboratory

Chapter 8

Introduction to JavaScript

Prepared by DT. Binh and H. Choo

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What We've Learned So Far

How to write content for a webpage using HTML

How to add styles to a webpage using CSS and linking a CSS file to

an HTML file

How to inspect the HTML and CSS of web pages in the browser

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Today: JavaScript

Now that we know how to add content and styles to a web page, let's

explore how to add responsive behavior

This lecture we'll cover the basics of the JavaScript language:

JavaScript vs. Java comparisons

Syntax and types

Loops and functions

Events (basics) and form elements

Later we'll use these building blocks to learn how to dynamically

update what you see on a web page in response to clicks, text input,

timers, etc.

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Terminology: Client-Side Scripting

Client-side script: Code runs in browser after page is sent back from server. Often, this code manipulates the page or responds to user actions.

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What is JavaScript?

A lightweight programming language ("scripting language")

Created in 1995 (over 10 days) by Brendan Elch (originally called

Mocha then LiveScript)

Used to make web pages interactive:

Insert dynamic text into HTML (ex: username)

React to events (ex: page load, user's mouse click)

Get information about a user's computer (ex: what browser they are using)

Perform calculations on user's computer (ex: form validation)

A web standard (but not supported identically by all browsers)

NOT related to Java other than name and some syntactic similarities...

Can be used in the browser, Adobe Acrobat, Adobe Photoshop,

embedded computers, the Unix terminal, etc. (we will be using it in the

browser)

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JavaScript vs. Java

Interpreted, not compiled

More relaxed syntax and rules

Fewer and "looser" data types

Variables don't need to be declared

Errors often silent (few exceptions)

JavaScript's key construct is the function rather than the object/class

"first-class" functions are used in many situations

Contained within a web page and integrates with its HTML/CSS

content

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Linking to a JavaScript file: <script>

The script tag should be placed in the HTML page's head

Script code is stored in a separate .js file

JS code can be placed directly in the HTML file's body or head (like

CSS)

This is bad style though: You should always separate content,

presentation, and behavior

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Our First JavaScript Statement: alert

A JS command that pops up a dialog box with a message

Syntax:

Example:

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Variables and types

Syntax:

Example:

Variables are declared with the let keyword (case-sensitive). You may also see

var used instead of let - this is an older convention, and you should use let in this

class.

Types are not specified, but JS does have types ("loosely-typed")

• Number, Boolean, String, Array, Object, Function, Null, Undefined

• Can find out a variable's type by calling typeof

• Note: Type conversion isn't always what you expect...

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Number type

Integers and real numbers are the same type (no int vs. double)

Same operators: + - * / % ++ -- = += -= *= /= %=

Similar precedence to Java

Many operators auto-convert types: "2" * 3 is 6

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String type

Methods: charAt, charCodeAt, fromCharCode, indexOf,

lastIndexOf, replace, split, substring, toLowerCase, toUpperCase

charAt returns a one-letter String (there is no char type)

length is a property (not a method, as it is in Java)

Concatenation with +: 1 + 1 is 2, but "1" + 1 is "11"

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More about Strings

Escape sequences behave as in Java: \' \" \& \n \t \\

To convert between numbers and Strings:

let count = 10; // 10

let stringedCount = "" + count; // "10"

let helloCount = count + " hello!"; // "10 hello!"

let magicNum = parseInt("42 is the answer"); // 42

let mystery = parseFloat("Am I a number?"); // NaN

To access characters of a String s, use s[index] or s.charAt(index):

let firstLetter = helloCount[0]; // "1"

let fourthLetter = helloCount.charAt(3); // “h"

let lastLetter = helloCount.charAt(helloCount.length - 1); // "!"

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Comments (same as Java)

Identical to Java's comment syntax

Recall: 3 comment syntaxes

HTML: <!-- comment -->

CSS/Java/JS: /* comment */

Java/JS: // comment

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for loop (same as Java)

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Math object

Methods: abs, ceil, cos, floor, log, max, min, pow, random, round, sin,

sqrt, tan

Properties: E, PI

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Logical Operators

Relational: > < >= <=

Logical: && || !

Equality: == != === !==

Most logical operators automatically convert types. These are all true:

5 < "7"

42 == 42.0

"5.0" == 5

The === and !== are strict equality tests; checks both type and value: "5.0"

=== 5 is false

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Boolean Type

Any value can be used as a Boolean

"falsey" values: 0, 0.0, NaN, "", null, and undefined

"truthy" values: anything else

Converting a value into a Boolean explicitly:

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Special Values: null and undefined

undefined: has not been declared, does not exist

null: exists, but was specifically assigned an empty or null value

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if/else Statements (same as Java)

Identical structure to Java's if/else statements

JavaScript allows almost anything as a condition

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while loops (same as Java)

break and continue keywords also behave as in Java but do not use them in

this class!

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Arrays

Two ways to initialize an array

length property (grows as needed when elements are added)

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Array methods

Array serves as many data structures: list, queue, stack, ...

Methods: concat, join, pop, push, reverse, shift, slice, sort, splice, toString, unshift

• push and pop add/remove from back

• shift and unshift add/remove from front

• shift return the element that is removed

• pop return the first element (not removed)

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Splitting strings: split and join

• split breaks apart a String into an array using a delimiter

• Can also be used with regular expressions surrounded by /:

▪ let a = s.split(/[ \t]+/);

• join merges an array into a single String, placing a delimiter between them

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Defining functions

The above could be the contents of example.js linked to our HTML page

Statements placed into functions can be evaluated in response to user events

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Event-Driven Programming

Unlike Java programs, JS programs have no main; they respond to user actions

called events

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Event Handlers

JavaScript functions can be set as event handlers

When you interact with the element, the function will execute

onclick is just one of many event HTML attributes we'll use

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Buttons: <button>

Button's text appears inside tag; can also contain images

To make a responsive button or other UI control:

• Choose the control (e.g., button) and event (e.g., mouse click) of interest

• Write a JavaScript function to run when the event occurs

• Attach the function to the event on the control

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Accessing an Element:

document.getElementById returns the DOM object for an element with a given

id (note that you omit the # when giving an id)

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document.getElementById: An Example

Practice on your computer!

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So... how can JavaScript be useful on a

webpage?

A common use for JavaScript on webpages is for event-handling and

form validation. To use form validation, we need to learn about a few

more HTML tags...

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Form Elements: <input>

Practice on your computer

name attribute specifies name of query parameter to pass to server

type can be button, checkbox, file, hidden, password, radio, reset, submit,

text, ...

value attribute specifies control's initial text

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<input> text fields

Practice on your computer!

input attributes: disabled, maxLength, readonly, size, value

size attribute controls onscreen width of text field

maxlength limits how many characters the user is able to type into the field

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Form Elements: Text boxes: <textarea>

Practice on your computer!

Initial text is placed inside textarea tag (optional)

Required rows and cols attributes specify height/width in characters

optional readonly attribute means text cannot be modified

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Resources and Tips

Review programming basics: using variables, arrays, loops, if-stateme

nts, and functions

Go over some JavaScript tutorials - there are many great ones!

https://developer.mozilla.org/en-US/docs/Learn/Getting_started_with_the_

web/JavaScript_basics

https://www.codecademy.com/learn/introduction-to-javascript

Practice! We now have JavaScript problems on Practice-It (80+ proble

ms) and its sister site CodeStepByStep (200+ problems).

Check out cool examples of JavaScript on the web!

Dennis Music Video

Robby Leonardi Mario-style Resume

Species in Pieces

Rainbow Worm

OMFGDOGS