Javascript II

Expressions andData Types

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JavaScript Review

programs executed by the web browser programs embedded in a web page

using the script element programs consist of statements

executed sequentially

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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");

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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

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Variable declaration

Declaration is optional in Javascript Syntax

var foo;

Says that foo is a variable we will use later

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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

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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

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JavaScript Types

Strings Integers Floats

real numbers Boolean

true or false

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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);

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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

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Examples

val1 = 45; val2 = 055; val3 = val1 + val2; Math.sqrt(val3);

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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

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Examples

val1 = 5.3; val2 = 5.3e1; val3 = val1 + val2; Math.floor(val3);

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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

<script type="text/javascript">

var foo;

document.write (foo);

</script>

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

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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

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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);

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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;

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The "+" trap

+ means different things for different types "foo" + "bar" "foobar" "5" + "6" "56" 5 + 6 11

What about? "5" + 6 6 + "foo"

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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!

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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

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Predefined Functions

You can define your own functions later in the class

Many built-in prompt document.write

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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

function name parameter #1 parameter #2

parameter list