javascript objects

Objects
● An object is an unordered collection of properties, each of
which has a name and a value.
● Property names are strings, so objects map strings to
values.
● In addition to maintaining its own set of properties, a
JavaScript object also inherits the properties of its
“prototype” object.
● JavaScript objects are dynamic—properties can usually be
added and deleted
● Objects are mutable and are manipulated by reference
rather than by value.

Creating Objects
● Objects can be created with object literals, with the new keyword,
and (in ECMAScript 5) with the Object.create() function.
● An object literal is a comma-separated list of colon-separated
properties (name:value pairs), enclosed within curly braces
●
A property name is a JavaScript identifier or a string literal (the
empty string is allowed).
● A property value is any JavaScript expression; the value of the
expression (it may be a primitive value or an object value)
becomes the value of the property.

Creating Objects
● Examples
● var empty = {}; // An object with no properties
● var point = { x:0, y:0 }; // Two properties
● // With more complex properties
var point2 = { x:point.x, y:point.y+1 };
● // Nonidentifier property names are quoted
var book = { "main title": "JavaScript", // space in property nam
'sub-title': "Pocket Ref", // punctuation in name
"for": "all audiences", // reserved word name
};

Creating Objects with new
● The new operator creates and initializes a new
object.
● The new keyword must be followed by a
function invocation (such functions is called a
constructor and serves to initialize a newly
created object).
● Core JavaScript includes built-in constructors
for native types, own constructor functions can
be defined to initialize newly created objects.

Creating Objects with new
● Examples
– var o = new Object(); // An empty object: same as
{}.
– var a = new Array(); // An empty array: same as [].
– var d = new Date (); //A Date for the current time.
– var r = new RegExp("js"); // A pattern matching
object.

Prototype Object
● Every Java-Script object has a second JavaScript
object (or null, rarely) associated with it, called as
Prototype, from which the properties are inherited
● All objects created by object literals have the same
prototype object refered in code :
Object.prototype
● Objects created using the new keyword and a
constructor invocation use the value of the
prototype property of the constructor function
as their prototype.

Prototype Object
● Object created by new Object() inherits from Object.prototype
just as the object created by {} does.
● The object created by new Array() uses Array.prototype as its
prototype, and the object created by new Date() uses
Date.prototype as its prototype.
● Object.prototype is one of the rare objects that has no
prototype: it does not inherit any properties.
● All other prototype objects inherit from Object.prototype object, so
the objects created using other objects like Date(), Array() etc
inherit from both the corresponding prototype object and
Object.prototype (prototype chain).

Creating Objects using
Object.create()
● Object.create(), creates a new object, using
its first argument as the prototype of that
object.
● Object.create() also takes an optional second
argument that describes the properties of
the new object.
● Object.create() is a static function, not a
method invoked on individual objects.

Creating Objects using
Object.create()
● Example
– // o1 inherits properties x and y.
var o1 = Object.create({x:1, y:2});
– // o2 inherits no properties or methods, no basic
methods will works as nothing is inherited
var o2 = Object.create(null);
– // o3 is like {} or new Object().
var o3 = Object.create(Object.prototype);

Objects
● Objects have attributes and methods.
● Many pre-defined objects and object types
exist.
● Using objects follows the syntax of C++/Java:
– objectname.attributename
– objectname.methodname()

The document object
● Many attributes of the current document are
available via the document object:
– Title
– Referrer
– URL
– Images
– Forms
– Links
– Colors

document Methods
● document.write() – the output goes into the
HTML document.
– document.write("My title is" + document.title);
● document.writeln() - adds a newline after
printing.

The navigator Object
● Represents the browser and is a Read-only
● Attributes include:
– appName
– appVersion
– platform

navigator Example
if (navigator.appName == "Microsoft Internet
Explorer") {
document.writeln("<H1>This page
requires Netscape!</H1>");
}

The window Object
● Represents the current window.
● There are possible many objects of type
Window, the predefined object window
represents the current window.
● Access to, and control of, a number of
properties including position and size.

window attributes and methods
● Attributes
– document
– name
– Status ( the status
line)
– Parent
● Methods
– Alert()
– Close()
– Prompt()
– MoveTo()
– MoveBy()
– Open()
– scroll()
– ScrollTo()
– resizeBy()
– resizeTo()

String Object
● A string is an immutable ordered sequence of 16-bit
values, each of which represents a Unicode
character - strings are JavaScript’s type for
representing text.
● JavaScript does not have a special type that
represents a single character of a string – To
represent a single 16-bit value, we need to use a
string that has a length of 1.
● Double-quote characters may be contained within
strings delimited by single-quote characters, and
single-quote characters may be contained within
strings delimited by double quotes

String Object
● "" // The empty string: it has zero characters;
● ' name="myform" '
● "Wouldn't you prefer O'Reilly's book?"
● "This stringnhas two lines"
● "π = 3.14"

Escape Sequences
● Backslash () is used to represent escape
sequences
● xYY - The Latin-1 character rep by two
hexadecimal digits XX
● uXXXX - The Unicode character specified by
the four hexadecimal digits XXXX

JavaScript escape sequences
●
Sequence Character represented
●
0 The NUL character (u0000)
●
b Backspace (u0008)
●
t Horizontal tab (u0009)
● n Newline (u000A)
● v Vertical tab (u000B)
● f Form feed (u000C)
● r Carriage return (u000D)
● " Double quote (u0022)
● ' Apostrophe or single quote (u0027)
● Backslash (u005C)

String Concatenation & String
Length
● String Concatenation is supported by +
operator
● msg = "Hello, " + "world"; // => "Hello, world"
● To determine the length of a string, the
number of 16-bit values it contains, is retrieved
using the length property of the string -
s.length

String Methods
● var s = "hello, world"
● s.charAt(0)
● s.charAt(s.length-1)
● s.substring(1,4)
● s.slice(1,4)
● s.slice(-3)
● s.indexOf("l")
● s.lastIndexOf("l")
● s.indexOf("l", 3)
● s.split(", ")
● s.replace("h", "H")
● s.toUpperCase()
● //Start with some text as example
● => "h": the first character.
● => "d": the last character.
● => "ell": chars 2, 3, and 4
● => "ell": same thing
● => "rld": last 3 characters
● => 2: position of first l.
● => 10: position of last l.
● => 3: position at or after 3
● => ["hello", "world"]
● => "Hello, world":
● replaces all instances
● => "HELLO, WORLD"

Immutable Property of Strings
●
Strings are immutable in JavaScript.
●
Methods like replace() and toUpperCase() return new strings: they
do not modify the string on which they are invoked.
●
In ECMAScript 5, strings can be treated like read-only arrays, and
we can access individual characters (16-bit values) from a string
using square brackets instead of the charAt() method:
●
s = "hello, world";
●
s[0] // => "h"
●
s[s.length-1] // => "d"

String to Numbers Conversion
● Global Functions
– parseInt()
● parses only integers
● If a string begins with “0x” or “0X,” parseInt() interprets it as a hexadecimal
number.
● It accepts an optional second argument specifying the radix (base) of the
number to be parsed. (2 to 36)
– parseFloat() - parses both integers and floating-point numbers.
– Both functions skip leading whitespace, parse as many numeric
characters as they can, and ignore anything that follows.
– If the first nonspace character is not part of a valid numeric literal,
they return NaN

String to Numbers Conversion
● Examples
– parseInt("3 blind mice") // => 3
– parseFloat(" 3.14 meters") // => 3.14
– parseInt("-12.34") // => -12
– parseInt("0xFF") // => 255
– parseFloat("$72.47"); // => NaN
– parseInt("11", 2); // => 3 (1*2 + 1)
– parseInt("077", 8); // => 63 (7*8 + 7)
– parseInt("ff", 16); // => 255 (15*16 + 15)

Number Conversion to String
● var n = 123456.789;
● n.toFixed(2); // "123456.79"
● n.toExponential(3); // "1.235e+5"
● n.toPrecision(7); // "123456.8"

toString()
● The toString() method defined by the Number class
accepts an optional argument that specifies a radix, or
base, for the conversion. If not specified the conversion is
done in base 10.
● Example
– var n = 17;
– binary_string = n.toString(2); // Evaluates to "10001"
– octal_string = "0" + n.toString(8); // Evaluates to "021"
– hex_string = "0x" + n.toString(16); // Evaluates to "0x11"

Math Object
● JavaScript supports more complex
mathematical operations through a set of
functions and constants defined as properties
of the Math object

Math functions and Properties
● Math.pow(2,53) //=> 9007199254740992: 2 to the power 53
● Math.round(.6) //=> 1.0: round to the nearest integer
● Math.ceil(.6) //=> 1.0: round up to an integer
● Math.floor(.6) //=> 0.0: round down to an integer
● Math.abs(-5) //=> 5: absolute value
● Math.max(x,y,z) //Return the largest argument
● Math.min(x,y,z) //Return the smallest argument
●
Math.random() //Pseudo-random number 0 <= x < 1.0
● Math.PI // π
● Math.E // e: The base of the natural logarithm

Math functions and Properties
● Math.sqrt(3) // The square root of 3
● Math.pow(3,1/3) // The cube root of 3
● Math.sin(0) // Trig: also Math.cos, Math.atan, etc.
● Math.log(10) // Natural logarithm of 10
● Math.log(100)/Math.LN10 // Base 10 logarithm of 100
● Math.log(512)/Math.LN2 // Base 2 logarithm of 512
● Math.exp(3) // Math.E cubed

Infinity
● Arithmetic in JavaScript does not raise errors in cases of
overflow, underflow, or division by zero.
● When the result of a numeric operation is larger than the
largest representable number (overflow), the result is
Infinity.
● Similarly, when a negative value becomes larger than the
largest representable negative number, the result is negative
infinity, printed as -Infinity.
● When adding, subtracting, multiplying, or dividing Infinity by
anything results in an infinite value (possibly with the
sign reversed).

NaN
● The not-a-number value has a feature in JavaScript: it does not
compare equal to any other value, including itself.
● x == NaN to determine whether the value of a variable x is NaN.
Instead, you should write x != x, returns true if, and only if, x is
NaN.
●
The function
– isNaN() - returns true if its argument is NaN, or if that argument is
a nonnumeric value such as a string or an object.
– isFinite() returns true if its argument is a number other than NaN,
Infinity, or -Infinity

Type Conversions
● Implicit Conversion
– Boolean conversion as false for :
● undefined
● null
● 0
● -0
● NaN
● "" // the empty string
– Boolean conversion to true - All other values, including
all objects (and arrays)

Type Conversions
● Implicit Conversion - Examples
– 10 + " objects" // => "10 objects". 10 -> string
– "7" * "4" // => 28: both strings -> numbers
– var n = 1 – "x"; // => NaN: "x" can't convert to a number
– n + " objects" // => "NaN objects": NaN -> "NaN"
– Following comparisons are true, after conversion
● null == undefined //These two are treated as equal.
● "0" == 0 // String -> a number before comparing.
● 0 == false //Boolean -> number before comparing.
● "0" == false //Both operands -> 0 before comparing.

Type Conversions
●
Implicit Conversion
●
If one operand of the + operator is a string, it converts the other
one to a string.
●
The unary + operator converts its operand to a number.
●
And the unary ! operator converts its operand to a boolean and
negates it.
●
Examples
– x + "" // Same as String(x)
– +x // Same as Number(x).
– x-0 // Same as Number(x).
– !x // Same as Boolean(x)

Type Conversions
● Explicit Conversion – done using the
Boolean(), Number(), String(), or Object()
functions:
– Number("3") // => 3
– String(false) // => "false" Or false.toString()
– Boolean([]) // => true
– Object(3) // => new Number(3)

Undeclared Variables
● Assigning a value to an undeclared variable,
Java-Script actually creates that variable as a
property of the global object, and it works
like a properly declared global variable
● Best Practise – to declare the variables using
'var'

Array Objects
● Arrays are supported as objects.
● Attribute length
● Methods include: concat, join, pop, push,
reverse, sort

EIW: Javascript the Language 42
Some similarity to C++
• Array indexes start at 0.
• Syntax for accessing an element is the
same:
a[3]++;
b[i] = i*72;

New in JS
• Arrays can grow dynamically – just add
new elements at the end.
• Arrays can have holes, elements that
have no value.
• Array elements can be anything
– numbers, strings, or arrays!

Creating Array Objects
• With the new operator and a size:
var x = new Array(10);
• With the new operator and an initial set
of element values:
var y = new Array(18,”hi”,22);
• Assignment of an array literal
var x = [1,0,2];

Arrays and Loops
var a = new Array(4);
for (i=0;i<a.length;i++) {
a[i]=i;
}
for (j in a) {
document.writeln(j);
}

Array Example
var colors = [ “blue”,
“green”,
“yellow];
var x = window.prompt(“enter a
number”);
window.bgColor = colors[x];

Array of Arrays
• Javascript does not support
2-dimensional arrays (as part of the
language).
• BUT – each array element can be an
array.
• Resulting syntax looks like C++!

Array of Arrays Example
var board = [ [1,2,3],
[4,5,6],
[7,8,9] ];
for (i=0;i<3;i++)
for (j=0;j<3;j++)
board[i][j]++;

javascript objects

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