Ch. 17 FIT5, CIS 110 13F

Chapter 17
Fundamental Concepts Expressed in JavaScript
Wednesday, April 9, 14

Copyright © 2013 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Programming Concepts
• Programming is the act of writing an
algorithm or program
• JavaScript is an embedded programming
language
– JavaScript lives in a <script> element in a web
page

JavaScript Runner Page
We can test JavaScript statements here:
http://www.headfirstlabs.com/books/hfjs/
hfjs_handson.php
you can type in JavaScript code and run it
directly in your web browser. You don't
even have to create a file—just enter your
code and click the Run button.

JavaScript Variables
• In programming terminology, variables are
storage locations that hold values and
have a name
• The values can change => vary
• To change the value of a variable:
– assignment statement

Variable Identifier Rules
• a variable’s name is called an identifier
• must begin with a letter, followed by any
sequence of letters, numerals, or the
underscore symbol
• Identifiers are not allowed to contain
spaces, punctuation, etc.
• Identifiers are case sensitive

Variable Declaration Statement
var area, radius;
• This command declares that two identifiers
(area, radius) will be used as variables
• Every variable used must be declared
• Always use the var keyword

Statement Terminator (;)
• A program is simply a list of statements;
• Each statement must be terminated by a
semicolon;
• JavaScript provides a feature called
Automatic Semicolon Insertion (ASI);
• There are rarely problems in omitting
semicolons;
• Best practice:
always include semicolons;

Undefined Values
• The declaration states that the identifier is
the name of a variable
• The name has no value at first, it is not
defined
• It is a name that doesn’t name anything
• The name is declared but there is no value
assigned yet
– The value is undefined

Initializing a Declaration
• Declaring variables with initial values
var taxRate = .088;
var balanceDue = 0;
var dayOfWeek;
• dayOfWeek has a special value
The value is undefined
• This works, too:
var taxRate = .088, balanceDue = 0;

Three Basic Data Types of
JavaScript
• There are three types of data in JavaScript
programs that will be used in this book:
1. numbers,
2. strings (a.k.a. text)
3. booleans

Numbers
2
3.14159
-23
4900000000
Math.random();

Strings
• “Abc”
• “Z”
• “”
• If our string contains single quotes, enclose
it in double quotes:
var book = "That’s a great quote!";
– quote swapping

Boolean Values
• There are only two Boolean values:
true and false

The Assignment Statement
• If variables are to change values in an
algorithm or program, there should be a
command to do so
• The assignment statement changes a
variable’s value
• An assignment statement has three parts
that always occur in this order:
<variable> <assignment symbol> <expression>;

Assignment Symbol
• Different programming languages use
different symbols for indicating assignment
• The three most widely used symbols are:
– The equal sign (=)
– The colon/equal sign pair (:=)
– The left pointing arrow (←)
• An example of an assignment is:

Interpreting an Assignment
Statement
• To understand how assignment works, you
must think of a value flowing from the right
side (expression side) to the left side
(variable side)
• The assignment symbol should be read as
“is assigned ” or “becomes” or “gets”
• These terms emphasize the role that the
assignment symbol plays in changing the
value of the variable named

Interpreting an Assignment
Statement
• example
var days = 44;
var weeks = days / 7;
alert(weeks);

An Expression and Its Syntax
• expressions are anything that give you a value
• most expressions are algebra-like formulas
• Expressions are built of variables and operators:
– addition (+) and subtraction(–)
– multiplication (*) and division (/ )
• These are called the arithmetic operators

Arithmetic Operators
+, -, *, /, %
• The modulus (mod) operation (%) divides
two integers and returns the remainder
– The result of a % b is the remainder of integer
division a / b
– Examples:
4 % 2 => 0
5 % 2 => 1

Relational Operators
• Relational operators make
comparisons between numerical values
• The outcome of the
comparison is a Boolean
value of true or false

Logical Operators: &&
• Logical AND
– The && is the logical AND operator
– The outcome of a && b is true if both a and b
are true; otherwise, it is false
var class = “199”, today = “Fri”;
alert((class == “110”) && (today == “Wed”))
//this alert displays false

Logical AND

Logical Operators: ||
• Logical OR
– The outcome of a || b is true if:
• either a is true or b is true
• if they are both true
– It is false only if both are false
• && and || have lower precedence than the
relational operators
– Relational operators are always evaluated before logic
operators

Logical Operators: !
• logical NOT (!)
– It is a unary operator, taking only a single
operand
– Its outcome is the opposite of the value of its
operand
– This alert will display true
var class = “CIS 110”;
alert(!(class == “CIS 199”));

Operator Overload (+)
• Operator overload is a technical term
meaning the “use of an operator with
different data types”
• Operators usually apply to a single data
type
– 4 + 5 produces the numerical result of 9
• If operands are strings, what does the
+ mean?

Concatenation
• + joins strings => concatenation
• The meaning of + is overloaded:
2 + 2 => 4, a number
“2” + “2” => “22”, a string

Conditional Statement: IF
var waterTemp = parseInt(prompt(‘enter temp’));
if (waterTemp < 32)
waterState = "Frozen";

if/else Statements
if (<Boolean expression>)
<then-statement>;
else
<else-statement>;

if with else
if (waterTemp < 32)
else
waterState = " Liquid ";
alert(" state = " + waterState);

Statement Block: { }
if (waterTemp < 32) {
}
else {
waterState = " Liquid ";
}

WaterStates Problem
Write a JavaScript that recognizes the
states of water
Embed your script in a web page

Ch. 17 FIT5, CIS 110 13F

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Ch. 17 FIT5, CIS 110 13F