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LBSC 690: Session 10
Programming, JavaScript
Jimmy Lin
College of Information Studies
University of Maryland
Monday, November 12, 2007
Software
Software “does something”
Tells the machine how to operate on some data
Software models aspects of reality
Input and output represent the state of the world
Software describes how the two are related
Examples:
Ballistic computations
Visa’s credit card verification system
Google
Microsoft Word
Types of Software
Application programs (e.g., PowerPoint)
What you normally think of as a “software’’
Compilers and interpreters
Programs used to write other programs
Operating system (e.g., Windows XP)
Manages computing resources
Embedded software (e.g., TiVO)
Programs permanently embedded inside some physical
device
Programming Languages
Software “does something”
Programming languages tell the machine “what to do”
Special purpose: geared towards specific tasks
Complex math (e.g., Matlab)
Databases (e.g., SQL)
General purpose: able to accomplish anything
Examples: Java, JavaScript, Perl, C, C++, ...
Types of Programming
Machine language
Language that the machine can directly understand
Assembly language
Directly specifies actions of the machine
Assembler changes instructions to machine code
High-level languages
Specifies machine instructions at a more abstract level
Compiler/interpreter translates to machine language
Examples: C/C++, Java, JavaScript
Visual programming languages
Visually arrange interface components
Example: Visual Basic
Types of Languages
Hardware
Machine Language
Assembly Language
C Java Pascal COBOL …
Machine Language
Everything is a binary number
Operations
Data
Memory locations
For instance
00001000 ADD
00010101 first number (21)
01010110 second number (86)
0010 register 2
Assembly Language
One level up from machine language
Symbolic instructions
Symbols representing memory locations
For instance
Assembly code is directly translatable into machine language
ADD 21, 86, R
High-Level Languages
Instructions represent higher-level constructs
Closer to how humans approach problems
Must be converted into machine code by a compiler or
interpreter
Programming: Overview
A program consists of a sequence of instructions
Basic concepts:
Data types
Variables
Basic operations
Instructions
Structures for controlling how instructions are executed:
Sequential
Conditional
Repetition
Programming: Foundations
Data types = things that you can operate on
Boolean: true, false
Number: 5, 9, 3.
String: “Hello World”
Variables hold values of a particular data type
Represented as symbols (e.g., x )
Operations = things that you can do
- x reverse the sign of x (negation)
6+5 Add 6 and 5 (numeric)
“Hello” + “World” Concatenate two strings
2.1 * 3 Multiply two values
x ++ increase value of x by 1
Basic Instructions
Assignment = store the result of an operation
x = 5 set the value of x to be 5
x += y x = x + y
x *= 5 x = x * 5
In JavaScript, var declares a variable
var b = true; create a boolean b and set it to true
var n = 1; create a number n and set it to 1
var s = “hello”; create a string s and set it to “hello”
In JavaScript, all instructions end with a semicolon (;)
Functions
Reusable code for doing a single task
A function takes in one or more parameters and returns one value
function convertToCelsius(f) {
var celsius = 5/9 * (f-32);
return celsius;
function weirdAddition(a, b) {
var result = a + b - 0.5;
return result;
Calling Functions
When you “call” a function, you invoke the set of instructions it represents
c = convertToCelsius(60);
function convertToCelsius(f) {
var celsius = 5/9 * (f-32);
return celsius;
More Examples
var f = 60; c = convertToCelsius(f);
r = weirdAddition(2, 4);
var a = 2; var b = 3; r = weirdAddition(a, b);
Programming Paradigms
Procedural Programming
Group instructions into meaningful functions
Examples: C, Pascal, Perl
Object oriented programming
Group “data” and “methods” into “objects”
Naturally represents the world around us
Examples: C++, Java
Algorithms
Derived from the name of the Persian mathematician Al-Khwarizmi
A sequence of well-defined instructions designed to accomplish a certain task
Programming for the Web
Common Gateway Interface (CGI) [Server-side]
User inputs information into a form
Form values passed to the sever via CGI
Program on the server generates a Web page as a
response
JavaScript [Client-side, interpreted]
Human-readable “source code” sent to the browser
Web browser runs the program
Java applets [Client-side, compiled]
Machine-readable “bytecode” sent to browser
Web browser runs the program
Not related to JavaScript (other than similarities in
syntax)
Where is the JavaScript?
JavaScript is usually kept in the section of an HTML document
…
Handling Events
When does code actually get executed?
Events:
User actions trigger “events”
Embedded in all modern GUIs
Event handlers are used to respond to events
Examples of event handlers in JavaScript
onMouseover: the mouse moved over an object onMouseout: the mouse moved off an object onClick: the user clicked on an object
Input and Output
How do you get information to/from the user?
Forms provide a method for accepting input and
displaying output
In HTML
Please enter a number:
The sum of all numbers up to the number above is
JavaScript code var num = eval(document.input.number.value); document.output.number.value = 10;
Reads in a value eval function turns it into a number
Changes the value in the textbox
JavaScript Resources
Google “javascript”
Tutorials: to learn to write programs
Code: to do things you want to do (“borrow”)
Books
Programming Tips
Details are everything!
Careful where you place that comma, semi-colon, etc.
Write a little bit of code at a time
Add a small new functionality, make sure it works, then
move on
Don’t try to write a large program all at once
Debug by outputting the state of the program
Print out the value of variables using document.write
Is the value what you expected?
