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Javascript II

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Javascript II Expressions and Data Types – PowerPoint PPT presentation

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Title: 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

10
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)

16
Examples
  • val1 true
  • val2 false
  • val3 val1 !val2

17
Another kind of value
  • ltscript type"text/javascript"gt
  • var foo
  • document.write (foo)
  • lt/scriptgt
  • What is the value of foo?

18
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"

20
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

22
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

24
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
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