06-global-objects

Global Objects

• Are the objects we have available in the global scope (primitive objects)

• We can divide them in 3 groups

  • Objects containers of data: Object, Array, Function, Boolean, Number
  • Objects of utilities: Math, Date, RegExp
  • Objects of errors: Error

##Object

• Object is the father of all the javascript objects, this is, every object inherites from him

• To create an empty object we can use:

  • The literal notation : var o = {}
  • The constructor function Object(): var o = new Object();

• Any object contains the following properties and methods:

  • The property o.constructor containing the constructor function
  • The method o.toString() that returns the object converted into text
  • The method o.valueOf() that returns the value of the object (usuarlly o)

>>> var o = new Object();
>>> o.toString()
“[object Object]”

>>> o.valueOf() === o
true

##Array

• To create arrays we can use:

  • Literal notation: var a = []
  • Constructor function Array(): var o = new Array();

• We can pass parameters to the constructor Array()

  • Several parameters: Will be assigned as elements in the array
  • A number: Will be considered as the size of the array

>>> var a = new Array(1,2,3,'four');
>>> a;
[1, 2, 3, "four"]

>>> var a2 = new Array(5);
>>> a2;
[undefined, undefined, undefined, undefined, undefined]

• Because of the arrays are objects, we also have available all the methods and properties of the father Object()

>>> typeof a;
"object"

>>> a.toString();
"1,2,3,four"
>>> a.valueOf()
[1, 2, 3, "four"]
>>> a.constructor
Array()

• Arrays have the property length

  • Returns the size of the array (number of elements)
  • We can modify it and change the size of the array

>>> a[0] = 1; 
>>> a.prop = 2; 

>>> a.length
1
>>> a.length = 5
5
>>> a
[1, undefined, undefined, undefined, undefined]

>>> a.length = 2;
2
>>> a
[1, undefined]

Arrays have a few interesting methods:

• push()
• pop()
• sort()
• join()
• slice()
• splice()

>>> var a = [3, 5, 1, 7, 'test'];

>>> a.push('new') 
6
>>> a 
[3, 5, 1, 7, "test", "new"]

>>> a.pop() 
"new"
>>> a 
[3, 5, 1, 7, "test"]

>>> var b = a.sort();
>>> b
[1, 3, 5, 7, "test"]
>>> a
[1, 3, 5, 7, "test"]

>>> a.join(' is not ');
"1 is not 3 is not 5 is not 7 is not test"

>>> b = a.slice(1, 3);
[3, 5]
>>> a
[1, 3, 5, 7, "test"]

>>> b = a.splice(1, 2, 100, 101, 102);
[3, 5]
>>> a
[1, 100, 101, 102, 7, "test"]

>>> more about arrays methods

##Function

• Functions are objects
• We can create functions with the constructor function Function() (althiugh this method is not recomended as it internally uses an eval() )

Example:

>>> function sum(a, b) {return a + b;};
>>> sum(1, 2)
3
>>> var sum = function(a, b) {return a + b;};
>>> sum(1, 2)
3
>>> var sum = new Function('a', 'b', 'return a + b;');
>>> sum(1, 2)
3

• Functions have the following properties:
  • The property constructor that contains a reference to the constructor function Function()
  • The property length that contains the number of parameters accepted by the function
  • The property caller (no standard) that contains a reference to the function that called that function
  • The property prototype that contains an object:
    • It's only useful when we use this function as a constructor
    • All objects created by a constructor function maintain a reference to its property prototype and can use its properties as if they're of their own

>>> function myfunc(a){ return a; }
>>> myfunc.constructor
Function()

>>> function myfunc(a, b, c){ return true; }
>>> myfunc.length
3

>>> function A(){ return A.caller; }
>>> function B(){ return A(); }
>>> B()
B()

var some_obj = { 
  name: 'Ninja', 
  say: function(){ 
    return 'I am a ' + this.name; 
  }
}
>>> function F(){} 
>>> typeof F.prototype 
"object"
>>> F.prototype = some_obj;
>>> var obj = new F();
>>> obj.name 
"Ninja"
>>> obj.say() 
"I am a Ninja"

• Functions have the following methods:
  • The method toString() that returns the source code of the function
  • The methods call() and apply() that execute methods of other objects especifying the context (especifying a different this)
    • These two methods do the same but accepting the arguments in a different format

>>> function myfunc(a, b, c) {return a + b + c;}
>>> myfunc.toString()
"function myfunc(a, b, c) { 
return a + b + c; 
}" 

var some_obj = { 
  name: 'Ninja', 
  say: function(who){ 
    return 'Haya ' + who + ', I am a ' + this.name; 
  }
}
>>> some_obj.say('Dude'); 
"Haya Dude, I am a Ninja"

>>> my_obj = {name: 'Scripting guru'};
>>> some_obj.say.call(my_obj, 'Dude'); 
"Haya Dude, I am a Scripting guru"

some_obj.someMethod.apply(my_obj, ['a', 'b', 'c']);
some_obj.someMethod.call(my_obj, 'a', 'b', 'c');

• Functions have available the object arguments that (besides length) have the property callee that contains a reference to the function called (to itself)

>>> function f(){return arguments.callee;}
>>> f() 
f()

( 
  function(count){ 
    if (count <= 5) { 
      console.log(count); 
      arguments.callee(++count); 
    } 
  }
)(1)

##Boolean

• The object Boolean is a container for a boolean type value We can create Boolean objects with te constructor function Boolean()

Example:

>>> var b = new Boolean();
>>> typeof b 
"object"
>>> typeof b.valueOf() 
"boolean"
>>> b.valueOf() 
false

• The function Boolean used as a normal function (without new) returns the value passed as a parameter converted to boolean

Example:

>>> Boolean("test") 
true
>>> Boolean("") 
false
>>> Boolean({}) 
true

##Number

• The function Number() can be used:
  • As a normal function to convert values to numbers
  • As a constructor function (with new) to create objects
• The number objects have the following methods available: toFixed(), toPrecision() y toExponential()

Example:

>>> var n = new Number(123.456)
>>> n.toFixed(1) 
"123.5"

>>> (12345).toExponential() 
"1.2345e+4"

• The method toString() of a number object allow us to convert any number to a specific base

Example:

>>> var n = new Number(255);
>>> n.toString(); 
"255"
>>> n.toString(10); 
"255"
>>> n.toString(16); 
"ff"
>>> (3).toString(2); 
"11"
>>> (3).toString(10); 
"3"

##String

• We can create String objects with the constructor function String()
  A String object is NOT a String primitive type of data (valueOf())

Example:

>>> var primitive = 'Hello';
>>> typeof primitive; 
"string"
>>> var obj = new String('world');
>>> typeof obj; 
"object"

>>> Boolean("") 
false
>>> Boolean(new String("")) 
true

• A string object is similar to an array of characters:
  • Every character has an indexed position
  • It has available the property length

Example:

>>> obj[0] 
"w"
>>> obj[4] 
"d"
>>> obj.length 
5

• Although the methods belong to the String object, we can use them directly in the primitive type of data string (the object is created internally)

Example:

>>> "potato".length 
6
>>> "tomato"[0] 
"t"
>>> "potato"["potato".length - 1] 
"o"

• The String objects have available the following methods:

  • toLowerCase() returns the string converted to lowercase

  • charAt() returns the character found at the specified position

  • indexOf() looks for a text string inside of another text string and returns the position where it was found

    • If it doesn't find anything it returns -1

  • lastIndexOf() starts the search from the end of the string

    • If it doesn't find anything it returns -1

  That's whay the proper way of checking if a string exists inside of another string is → if ( s.toLowerCase().indexOf('couch') !== -1 ) {...}

  • slice() returns a piece of the text string

  • split() transform the string in an array by using the specified character/s as a separator

  • concat() joins strings

>>> var s = new String("Couch potato");
>>> s.toUpperCase() 
"COUCH POTATO"
>>> s.toLowerCase() 
"couch potato"
>>> s.charAt(0); 
"C"
>>> s.indexOf('o') 
1
>>> s.lastIndexOf('o') 
11
>>> s.slice(1, 5) 
"ouch"
>>> s.split(" ") 
["Couch", "potato"]
>>> s.concat("es") 
"Couch potatoes"

##Math

• Math is an object with properties and methods useful for mathematical work It is NOT a constructor of other objects

• Some interesting Math methods are:

  • random() generates random numbers from 0 to 1

  • round(), floor() y ceil() round numbers

  • min() y max() return the minimum and the maximum of a serie of numbers passed as parameters

  • pow() y sqrt() returns the power and the square each one

##Date

• Date() is a constructor function that creates date objects We can create new Date objects by passing:
  • nothinga (will take the current date as default)
  • A date in string format
  • separated values that represent: Year, Month (0-11), Day (1-31), Hour (0-23), Minutes (0-59), Seconds (0-59) y Miliseconds (0-999)
  • A timestamp value

Example (Firebug shows the result of the toString method over a date object):

>>> new Date() 
Tue Jan 08 2008 01:10:42 GMT-0800 (Pacific Standard Time) 
>>> new Date('2009 11 12') 
Thu Nov 12 2009 00:00:00 GMT-0800 (Pacific Standard Time)
>>> new Date(2008, 0, 1, 17, 05, 03, 120) 
Tue Jan 01 2008 17:05:03 GMT-0800 (Pacific Standard Time)
>>> new Date(1199865795109) 
Wed Jan 09 2008 00:03:15 GMT-0800 (Pacific Standard Time)

• Some methods to work with date objects are:

  • setMonth() y getMonth() write and read the month in a date object each one of them (we have available the same kind of methods for year, day, hours, minutes, etc…)

  • parse() given a string, it returns its timestamp

  • UTC() produces timestamp given some month, year, day, etc..

  • toDateString() returns the content of a date object in american format

Example:

>>> var d = new Date();
>>> d.toString(); 
"Wed Jan 09 2008 00:26:39 GMT-0800 (Pacific Standard Time)"
>>> d.setMonth(2); 
1205051199562
>>> d.toString(); 
"Sun Mar 09 2008 00:26:39 GMT-0800 (Pacific Standard Time)"
>>> d.getMonth(); 
2

>>> Date.parse('Jan 1, 2008') 
1199174400000
>>> Date.UTC(2008, 0, 1) 
1199145600000

>>> var d = new Date(2012, 5, 20);
>>> d.getDay(); 
3
>>> d.toDateString(); 
"Wed Jun 20 2012"