程序代写代做代考 python database data structure Java SQL asp.net hbase javascript COMP284 Scripting Languages – Handouts

COMP284 Scripting Languages – Handouts

COMP284 Scripting Languages
Lecture 14: JavaScript (Part 1)

Handouts

Ullrich Hustadt

Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science

University of Liverpool

Contents

1 JavaScript
Motivation
Overview
Example

2 Types and Variables
Types
Variables
Typecasting
Comparisons

COMP284 Scripting Languages Lecture 14 Slide L14 – 1

JavaScript Motivation

JavaScript: Motivation

• PHP and Perl both allow us to create dynamic web pages
• In web applications, PHP and Perl code is executed on the web server

(server-side scripting)

• allows to use a website template that is instantiated using data stored in a
database

• ‘business logic’ is hidden from the user:
the code of an application is not visible to the user/client;
the user/client only has access to the HTML produced by the code

• not ideal for interactive web applications:
too slow to react and too much data needs to be transferred

• operations that refer to the location of the user/client are difficult,
for example, displaying the local time

echo date(’H:i l, j F Y’);

displays the local time on the server not the local time for the user

COMP284 Scripting Languages Lecture 14 Slide L14 – 2

JavaScript Overview

JavaScript

• JavaScript is a language for client-side scripting
• script code is embedded in a web page (as for PHP),

but delivered to the client as part of the web page and
executed by the user’s web browser
; code is visible to the user/client

• allows for better interactivity as reaction time is improved and
data exchange with the server can be minimised

• a web browser may not support JavaScript or
the user may have disallowed the execution of JavaScript code

• different JavaScript engines may lead to different results, in particular,
results not anticipated by the developer of JavaScript code

• performance relies on the efficiency of the JavaScript engine and
the client’s computing power (not the server’s)

• operations that refer to the location of the client are easy:

document.write(“Local time: ” + (new Date). toString ());

COMP284 Scripting Languages Lecture 14 Slide L14 – 3

JavaScript Overview

JavaScript: History

• originally developed by Brendan Eich at Netscape
under the name Mocha

• first shipped together with Netscape browser in September 1995
under the name LiveScript

• obtained its current name in December 1995 under a deal between
Netscape and Sun Microsystems, the company behind Java,
in December 1995

• does not have a particularly close relationship to Java,
it mixes aspects of Java with aspects of PHP and Perl
and its own peculiarities

• is a dialect of ECMAScript, a scripting language standardised in the
ECMA-262 specification and ISO/IEC 16262 standard since June 1997

• other dialects include Microsoft’s JScript and TypeScript and
Adobe’s ActionScript

COMP284 Scripting Languages Lecture 14 Slide L14 – 4

JavaScript Overview

Websites and Programming Languages

Website Client-Side Server-Side Database

Google JavaScript C, C++, Go, Java,
Python, PHP

BigTable, MariaDB

Facebook JavaScript Hack, PHP, Python,
C++, Java, . . .

MariaDB, MySQL,
HBase Cassandra

YouTube Flash,
JavaScript

C, C++, Python, Java,
Go

BigTable, MariaDB

Yahoo JavaScript PHP MySQL, PostgreSQL

Amazon JavaScript Java, C++, Perl Oracle Database

Wikipedia JavaScript PHP, Hack MySQL, MariaDB

Twitter JavaScript C++, Java, Scala MySQL

Bing JavaScript ASP.NET MS SQL Server
Wikipedia Contributors: Programming languages used in most popular websites. Wikipedia, The Free Encyclopedia,
20 October 2017, at 11:28. http://en.wikipedia.org/wiki/Programming_languages_used_in_most_popular_websites
[accessed 23 October 2017]

COMP284 Scripting Languages Lecture 14 Slide L14 – 5

http://en.wikipedia.org/wiki/Programming_languages_used_in_most_popular_websites

JavaScript Example

JavaScript: Hello World!

1 Hello World
2
3

Our first JavaScript script

4 7

10

• JavaScript code is enclosed between
• Alternative HTML markup that is to be used in case JavaScript is not

enabled or supported by the web browser, can be specified between

• File must be stored in a directory accessible by the web server, for
example $HOME/public_html, and be readable by the web server

• No particular file name extension is required

COMP284 Scripting Languages Lecture 14 Slide L14 – 6

JavaScript Example

JavaScript scripts

• JavaScript scripts are embedded into HTML documents and are
enclosed between