Javascript II

1
Javascript II

• Expressions andData Types

2
JavaScript Review

• programs executed by the web browser
• programs embedded in a web page
• using the script element
• programs consist of statements
• executed sequentially

3
Statements

• End with semi-colon
• Cannot occupy multiple lines
• Assignment statement
• var value
• The value on the right can be any legal
  expression
• Function call
• document.write ("foo")

4
Variables

• A variable is a named location where a value can
  be stored
• In JavaScript
• using a variable creates it
• can also declare
• Other languages
• require that variables be declared before being
  used
• Variables have "scope"
• locations in the program where they are valid
• will discuss this later on

5
Variable declaration

• Declaration is optional in Javascript
• Syntax
• var foo
• Says that foo is a variable we will use later

6
Data types

• What is the difference between the following
  statements
• val1 "53"
• val2 53
• These values are different to JavaScript
• stored differently
• manipulated differently
• same operation may mean something different
• on strings in concatenation
• on numbers is addition

7
Data Type

• A characterization of a stored value
• Determines
• what kinds of values can be stored
• how the value is stored internally
• what operations can be applied
• Syntactic representation
• how the value is expressed in a program

8
JavaScript Types

• Strings
• Integers
• Floats
• real numbers
• Boolean
• true or false

9
Strings

• What values
• lists of characters
• any length including the zero-length null
  string
• What operations
• concatenation ( operator)
• output to the Web page
• using document.write()
• returned by prompt() function
• Syntax
• double or single quotes

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Examples

• val1 Hello"
• val2 there'
• val3 val1 val2
• document.write (val3)
• val4 45
• val2 val3 val5
• document.write (val2)

11
Integers

• What values
• whole numbers between -9223372036854775808 and
  9223372036854775808
• What operations
• standard mathematical operations (, -, , /)
• special math functions
• Syntax
• unquoted integers
• Note
• book doesn't use integers in examples

12
Examples

• val1 45
• val2 055
• val3 val1 val2
• Math.sqrt(val3)

13
Float

• What values
• decimal values from 1.0x10308 to 1.0x10-323
• 17 digits of precision (past decimal point)
• What operations
• standard mathematical operations
• special math functions
• Syntax
• unquoted decimal values
• scientific notation
• 1.2e3 1.2 x 103 1200

14
Examples

• val1 5.3
• val2 5.3e1
• val3 val1 val2
• Math.floor(val3)

15
Boolean

• What values
• true / false
• What operations
• logical operations
• and
• or
• not !
• Syntax
• keywords (true, false)

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Examples

• val1 true
• val2 false
• val3 val1 !val2

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Another kind of value

• ltscript type"text/javascript"gt
• var foo
• document.write (foo)
• lt/scriptgt
• What is the value of foo?

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undefined value

• This is the value of a variable when it is
  created
• if you use a variable without defining it, you
  get an error
• You can declare a variable without giving it a
  value
• not an error
• variable has no value
• unexpected results

19
Expressions

• An expression is
• a legal combination of Javascript values,
  operators, function calls and variables
• that evaluates to a value
• Examples
• a b
• 5 / 6
• 3.14159 r r
• Math.sqrt (errorTerm)
• "foo" "-" "bar"

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Syntax alert

• An expression is not a statement
• an expression may be part of a statement
• note no semi-colon
• Example
• (a b) / 2
• expression
• size a b
• statement

21
Evaluation

• An expression is evaluated
• when it is executed (run-time)
• Steps
• Each variable is replaced by its current value
• Operators are applied
• Until a single value remains

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Example

• a 5
• b 20
• position (a 5) (b / 10)

23
Complex expression

• Expressions can be large and complex
• JavaScript doesn't care
• Readers of your program might care
• A complex expression can always be simplified
• by using intermediate variables

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Example

• complex expression
• slope ( (y1 y2) / (x1 x2) )
• simplified
• deltaY y1 y2
• deltaX x1 x2
• slope deltaY / deltaX

25
The "" trap

• means different things for different types
• "foo" "bar" ? "foobar"
• "5" "6" ? "56"
• 5 6 ? 11
• What about?
• "5" 6
• 6 "foo"

26
Operators

• is an operator
• An operator
• takes two values (sometimes one)
• makes ("returns") a new value
• Some operators are two characters
• Assignment is not an operation
• is not an operator!

27
Functions

• Like an operator
• takes values
• does some operation
• returns a result
• But
• has a name instead of symbol
• can take any number of values
• can be user-defined

28
Predefined Functions

• You can define your own functions
• later in the class
• Many built-in
• prompt
• document.write

29
Function syntax

• prompt ( "Enter a number", "0")
• return value is a string containing the user
  input
• No parameters?
• Math.random ()
• there's still a list, just an empty one

parameter 2
function name
parameter 1
parameter list