Javascript        

 Javascript  1              

 1 Core javascript  2              

 1.1 lexical structure   詞匯結(jié)構(gòu)          .            2              

 1.2 data types and values  2              

 1.2.1 numbers  3              

 1.2.3 strings  4              

 1.2.4 boolean values  6              

 1.2.5 functions  7              

 1.2.6 objects  7              

 1.2.7 arrays  8              

 1.2.8 null  9              

 1.2.9 undefined  9              

 1.2.10 the date object  9              

 1.2.11 regular expressions  10              

 1.2.12 error objects  10              

 1.2.13 type conversion summary  10              

 1.2.14 primitive data type wrapper objects  11              

 1.2.15 object-to-primitive conversion  12              

 1.2.16 by value versus by reference  13              

 1.3 variables;  16              

 1.3.1 variable declaration  16              

 1.3.2 variable scope  16              

 1.3.3 primitive types and reference types  17              

 1.3.4 garbage collection  17              

 1.3.5 variables as properties  17              

 1.4 expressions and operators  19              

 1.5 statements  22              

 1.6 objects and arrays  25              

 1.7 functions  29              

 1.7.1 basis  29              

 1.7.2 functions as data  30              

 1.7.3 functions as methods  32              

 1.7.4 as an object functions properties and methods  33              

 1.7.5 functions scope and closures  33              

 1.7.6 the Function() constructor  35              

 1.8 classes, constructors and prototypes  36              

 1.8.1 constructors  36              

 1.8.2 prototypes and inheritance  36              

   1 Core javascript    

   1.1 lexical structure       詞匯結(jié)構(gòu)    

 character set:        

Unicode character set(7-bit ASCII, 8-bit ISO Latin-1 encodings are subset of Unicode.)

 Case sensitivity:        

  A case sensitivity language.(html is not case sensitivity.)  

  Whitespace and line breaks:  

Js ignores spaces, tabs, new lines that appear between tokens.

 Optional semicolons:        

If your coding style is the same as java, then this rule can be ignored.

 Comments:        

Like java, js supports both c++(//) and c-style(/* */) comments.

 Literals:        

A literal is a data value that appears directly in a program. Following are all types of literals in js:

12    // The number twelve  

1.2    // The number one point two  

"hello world"    // A string of text  

'Hi'    // Another string  

true    // A Boolean value  

false    // The other Boolean value  

/javascript/gi    // A "regular expression" literal (for pattern matching)  

null    // Absence of an object  

  { x:1, y:2 }       // An object initializer

  [1,2,3,4,5]       // An array initializer

 identifiers:  

  you can use the same identifier naming ruls as in java, though there are some extra rules which can be ignored.

 Reserved words:        

   1.2 data types and values    

 primitive datatypes:        

  numbers,  

  string,  

  boolean,  

  null,  

  undetified,  

 composite datatypes:        

 object type:        

an  object(a member of object type)represents a collection of values(primitive values, composite values).

An objects can represent an unordered collection of named values or an ordered collection of named values. The latter one is called an  array.

Objects and array are the same datatype fundamentally, but they behave differently and will usually be considered distinct types.

Another special kind of object is  function. A function is an object that has executable code. A function has special behavior from other kind objects. Js defines a special syntax for function.

A few other kind of objectsstill exist. These objects don’t represent new data types, but new classes of objects, such as Date, RegExp, Error.

 1.2.1 numbers        

  In js, there is no distinction between integer and floating-point values. All numbers are represented as floating-point values using 64-bit floating-point format by IEEE754.  

  Number literal: a number that appears directly in js.  

 integer literals:  

you can exactly represent integers between -9007199254740992 (-2  ⁵³) and 9007199254740992 (2  ⁵³), inclusive. Note that certain integer operations in js are performed on 32-bit integers, which range from -2147483648 (-2  ³¹ ) to 2147483647 (2  ³¹-1).

 Hexadecimal and octal literals:        

   A hexadecimal literal begins with 0x or 0X.

Although the ECMAScript doesn’t support octal literals, some

implementations of javascript does. An octal literal begins with 0.  

 Floating-point literals:  

Floating-point literals can have traditional syntax such as

3.1415926, and also can be represented using exponential notation such as 1.444E-32.

Note that there are infinitely real numbers, but only a finite

number of them can be represented exactly by the js floating-point format.

  Working with numbers:        

  Arithmetic operators,  

  Mathematical functions,  

 Number conversions:        

  Numbers and strings can be interconverted .  

 Special numeric values:        

   Infinity,

NaN: not a number,

  Number.MAX_VALUE,  

  Number.MIN_VALUE,  

  Number.NaN: not a number,  

  Number.POSITIVE_INFINITY,  

  Number.NEGATIVE_INFINITY,  

 1.2.3 strings        

  a string is a sequence of Unicode letters, digits, punctuation characters, and so on.  

  A string is quoted by single or double quotation marks.  

  No char data type exists in js.  

  String literals:        

Double-quote characters can be contained within strings delimited by single quotation marks, vice versa.

  Escape sequences in string literals:        

   / is the escape character.

Note that the blackslash escape (/) can’t be used before a line break to continue a string token across two line or to include a literal line break in a string.

If the blackslash precedes any characters that is not in the table above it will be ignored simply. For an instance, /# is the same as #.

  Working with strings:        

  Concatenate strings by +:        

  Msg = “Hello, ” + “world!”;  

 Length of a string:        

  Msg.length;  

  More methods:  

  Msg.charAt( Msg.length -1 );  

  sub = s.substring(1,4);  

  i = s.indexOf('a');  

  more later.  

 Note that        

  last_char = s[s.length - 1];  

is not part of ECMAScript v3 standard, is not portable, and should be  avoided.

 String is not a data type of object,        

Though string properties and methods can be used as the same way as the object data type.

  Conversion between numbers and strings:        

 l        conversion-through-concatenation:

“a string”+100.

 l        String() function:

 l        toString(base) function:

primitive numbers are first convertedto Number object, then call the function above.

 l        toFixed(n)

display a number with specified digits (n) after the decimal point.

 l        toExponential(n)

specify a exponential number with one digit before the decimal point and n digits after the decimal point.

 l        toPrecision(n)

display a number using n digits of the primitive number.

Examples for above three:

var n = 123456.789;

n.toFixed(0);    // "123457"  

n.toFixed(2);    // "123456.79"  

n.toExponential(1);    // "1.2e+5"  

n.toExponential(3);    // "1.235e+5"  

n.toPrecision(4);    // "1.235e+5"  

n.toPrecision(7);    // "123456.8"  

 l        automatically conversion such as

var number = string_value – 0;

 l        Number():

Based 10 radix only works.

 l        parseInt(number):

 l        parsetInt(number, base):

 l        parseFloat():

 if the three functions above fail they return        NaN  .  

 1.2.4 boolean values        

  Boolean values can convert to and from other types and often      automatically converted        .  

  In a numeric context, true   à 1 and false   à 0.

  In a string context, true   à “true” and false   à “false”.

  When a boolean value needed a number ( 0 or NaN )   à false, else the number   à true. When a Boolean value needed en empty string   à false, else   à true. null and undefined value   à false, and non-null object, array, function   à true.

  Explicit conversion:  

  Boolean()  

  Another technique:  

  var x_as_boolean = !!x;  

 1.2.5 functions        

  An import feature of javascript which is different from other languages is that functions are values that can be manipulated by javascript code. That feature gives more flexibility.  

  Functions are values in js. It means that functions can be stored in variables, arrays and objects, and it means that functions can be passed as arguments(自變量，變量) to other functions. We can see details later.

  Function literals:  

  Three ways of defining a function:  

function square(x) { return x*x; }

var square = function(x) { return x*x; }

var square = new Function("x", "return x*x;");

 1.2.6 objects        

 An object is a collection of named values.         The values are referred as properties.  

 Properties         of objects are like javascript variables in many ways.  

  When a function value is stored in a property the function is called a      method        , such as document.write("this is a test");  

  Objects can serve as associative arrays. They can associate arbitrary data values with arbitrary strings. To use such objects, you should do it like this:  

  Image[“weight”],  

  Statement above can access a property value named weight.  

  Creating objects:        

      You can do it like :  

var o = new Object( );

var now = new Date( );

var pattern = new RegExp("//sjava//s", "i");

  then to use and set their properties:  

var point = new Object( );

point.x = 2.3;

point.y = -1.2;

  object literals:        

 an object literal syntax allows you to create an object and specify its properties, which is also called an object initiallizer.

You can implement  an object initiallizer as following:

  var point = { x:2.3, y:-1.2 };  

 nested object literals:        

var rectangle = { upperLeft: { x: 2, y: 2 },

  lowerRight: { x: 4, y: 4}  

  };  

Note:

  The properties in object literals can be arbitrary  javascript expressions besides constants. Also, the property names may be  strings       rather than identifiers:

var square = { "  upperLeft      ": { x:  point.x, y:  point.y },

 'lowerRight'      : { x:(  point.x + side), y:(  point.y+side) }};

  objects conversions:        

  non-null object        à         false,  

 object        à         object.toString(),

 object  à         object.valueOf(),if return object, use toString() and

try to convert the string value to a number.

 Object to primitive data type         will discuss later.  

 1.2.7 arrays        

  an array is a collection of data values just as an object is. While each data value contained in an object has a name, they has an index in an array. The first index is 0.  

  The data value in an array do not need to be the same data type as javascript is an untyped language.  

  Creating arrays:        

Method 1:

var a = new Array( );

a[0] = 1.2;

a[1] = "JavaScript";

a[2] = true;

a[3] = { x:1, y:3 };

   method 2:

  var a = new Array(1.2, "JavaScript", true, { x:1, y:3 });  

  method 3: create an array with 10 undefined elements,  

  var a = new Array(10);  

  array literals:        

  a literal syntax for creating and initializing arrays:        

  var a = [1.2, "JavaScript", true, { x:1, y:3 }];  

 nested array literals:        

  var matrix = [[1,2,3], [4,5,6], [7,8,9]];  

 elements in array literals can be arbitrary expressions:        

  var base = 1024;  

 var table = [base, base+1, base+2, base+3];  

 specify undefined elements:        

var sparseArray = [1,,,,5];

 1.2.8 null        

  null represents no object, a special value of object type.  

 1.2.9 undefined        

  undefined stands for a variable that has been declared but had no value assigned or an object property not exist.  

  Although undefined and null are not the same, == operator equal them. Because the two kind value usually indicate an absence of value. If you want to distinguish the above two you can use === or typeof operator.  

  Difference between null and undefined:  

  The latter is not a key word. ECMAScript v3 implement it by defining a global variable named undefined not assign a value in advance. You can implement that by defining one yourself ,just like “var undefined”.  

  void operator can return an undefined value too.  

 1.2.10 the date object        

  in order to represent date and time js provide a class of object. Examples are following:  

var now = new Date( );// Create an object holding the current date and time.

// Create a Date object representing Christmas.

// Note that months are zero-based, so December is month 11!

var xmas = new Date(2006, 11, 25);

there are methods to set and get the various date and time values and to convert the Date to string, using local time or GMT time. We will learn that in detail later.

 1.2.11 regular expressions        

  Regular expressions are used for pattern matching and for implementing search and replace operations.  

  Regular expressions are represented by the RegExp object.  

  RegExp can be created like Date using RegExp().  

  Also, RegExp can be create using a literal syntax as following:  

  /^HTML/  

/[1-9][0-9]*/

//bjavascript/b/i

 1.2.12 error objects        

  ECMAScript v3 defines a number of classes to represent errors.  

 1.2.13 type conversion summary        

 1.2.14 primitive data type wrapper objects        

  We can use values of the type string like an object as following:  

var s = "These are the times that try people's souls.";

var last_word = s.substring(s.lastIndexOf(" ")+1, s.length);

  But string is not object so what’s happen?  

  Js provide corresponding object classes for each of the three primitive data types, Number, String and Boolean classes.  

  Number, String, Boolean are      wrappers         which define properties and methods other than contain primitive values.  

  Note that the String object created when a string used in an object context is a transient one. It allows you to access a property or method, and then it is no longer needed, so reclaimed by the system. For example, var len = s.length;

  In this case,      s remains a string        . A new transient object is created to access the length property. And then the object is discarded with no change to the original value s.  

 Note that strings are automatically converted to String objects when needed.        Also, the reverse is true        . for examples,  

  msg = S + '!';  

  in the above case, S is a String object. It need to convert to a primitive string type in order to add with !.  

 Note that the discuss about string and String also applies to        number and Number, boolean and Boolean        .  

 An explicit way to use wrapper         as  

  var number_wrapper = Object(3);  

 1.2.15 object-to-primitive conversion        

  Something simple about this subject has been discussed in 1.3.6 . there are something more in detail.  

  Object   à number:

  Object.valueOf()  

  If not return a primitive number value call Object.toString(). And convert the string value to a number.  

  This leads to interesting results for arrays. The toString( ) method of arrays converts the array elements to strings, then returns the result of concatenating these strings, with commas in between. Therefore, an array with no elements converts to the empty string, which converts to the number zero! Also, if an array has a single element that is a number      n        , the array converts to a string representation of      n        , which is then converted back to      n         itself. If an array contains more than one element, or if its one element is not a number, the array converts to NaN.

  The conversion of a value depends on the context used.        

   Because +, <, <=, >, >= operate on both strings and numbers it is not clear whether to convert an object operator to a number or a string. In most cases, js first try to convert the object using valueOf(). If a primitive value( usually a number ) returned that value is used. Else js tries to convert the object to a string by calling toString().

  But there is an exception for Date. Because Date has both valueOf() and toString(). Sometimes you may want use a string, other times may use a number.  

  Most object either have no valueOf() or don’t have a valueOf() that returns an useful value. So they referred as strings.  

  Note that valueOf() don’t convert an object to a number, its job is converting an object to a primitive value, maybe sometimes a string.  

 1.2.16 by value versus by reference        

  there are three ways to manipulate a data value.  

firstcopy it.

  Second pass it as an argument to a function or method.  

  Third compare it with another value.  

  Primitive types are manipulated by value, and reference types are manipulated by reference. Objects are reference types. Numbers and booleans are primitive types. Strings are a little difference.  

  Objects contain arbitrary numbers of properties or elements, so manipulating they by value is not inefficient. Although a string has arbitrary length it is usually considered as primitive type as it is not an object. We will discuss about it later.  

  Copying, passing, comparing by value:        

  
 Copying, passing, and comparing by reference      
  

// Here we create an object representing the date of Christmas, 2007
// The variable xmas contains a reference to the object, not the object itself
var xmas = new Date(2007, 11, 25);
// When we copy by reference, we get a new reference to the original object
var solstice = xmas;// Both variables now refer to the same object value
// Here we change the object through our new reference to it
solstice.setDate(21);
// The change is visible through the original reference, as well
xmas.getDate( );// Returns 21, not the original value of 25
// The same is true when objects and arrays are passed to functions.
// The following function adds a value to each element of an array.
// A reference to the array is passed to the function, not a copy of the array.
// Therefore, the function can change the contents of the array through
// the reference, and those changes will be visible when the function returns.
function add_to_totals(totals, x)
{
  totals[0] = totals[0] + x;  
  totals[1] = totals[1] + x;  
  totals[2] = totals[2] + x;  
}
// Finally, we'll examine comparison by reference.
// When we compare the two variables defined above, we find they are
// equal, because they refer to the same object, even though we were trying
// to make them refer to different dates:
(xmas == solstice)// Evaluates to true
// The two variables defined next refer to two distinct objects, both
// of which represent exactly the same date.
var xmas = new Date(2007, 11, 25);
var solstice_plus_4 = new Date(2007, 11, 25);
// But, by the rules of "compare by reference," distinct objects are not equal!
(xmas != solstice_plus_4)// Evaluates to true
  
  Copying, passing, comparing strings:
  
  Because strings are immutable there is no way to tell whether strings are passed by value or by reference. You can assume that, for efficiency, JavaScript is implemented so that strings are passed by reference, but in actuality it doesn't matter, because it has no practical bearing on the code you write. That means that you can’t change a string whether it is used by reference or by value. so it does not matter which ways to use.
  
  Strings are compared by value. be careful for the code following, which shows that comparison is done by value while it means a comparison by reference.
  
// Determining whether strings are compared by value or reference is easy.
// We compare two clearly distinct strings that happen to contain the same
// characters. If they are compared by value they will be equal, but if they
// are compared by reference, they will not be equal:
var s1 = "hello";
var s2 = "hell" + "o";
if (s1 == s2) document.write("Strings compared by value");
    1.3 variables;  
  
  javascript is an untyped language. Java is strongly typed language which has the advantage of enforcing strict programming practices , which is suitable for long complex programs. The advantage of js is that it can conveniently and automatically convert values between different types. And codes in js is shorter, so simpler syntax is better.
  
 
  1.3.1 variable declaration      
  
  variables defined with var are permanent, which can’t be deleted with delete operator.
  
  If you assign a value to a variable you don’t declare with var, js implicitly declare it for you.
  
  Note that the implicit declaration always create a global variable, even if it is used within a body of a function.
  
  1.3.2 variable scope      
  
 unlike java and so on        , js 
      does not have block-level scope        . All variables declared in a function are defined throughout the function, no matter where they are declared. It means that in the following cods i, j, and k all have the same scope: throughout the body of function test.
  
function test(o) {
  var i = 0; // i is defined throughout function
  
  if (typeof o == "object") {
  
  var j = 0; // j is defined everywhere, not just block
  
  for(var k=0; k < 10; k++) { // k is defined everywhere, not just loop
  
  document.write(k);
  
  }
  
  document.write(k); // k is still defined: prints 10
  
  }
  
  document.write(j); // j is defined, but may not be initialized
  
}
 
 Another useful example:      
  
var scope = "global";
function f( ) {
  alert(scope); // Displays "undefined", not "global"
  
  var scope = "local";// Variable initialized here, but defined everywhere
  
  alert(scope); // Displays "local"
  
}
f( );
  1.3.3 primitive types and reference types      
  
  1.3.4 garbage collection      
  
  garbage collection is automatic and invisible to the programmer.
  
  1.3.5 variables as properties      
  
  variables are fundamentally the same as object properties.
  
  When the JavaScript interpreter starts up, one of the first things it does, before executing any JavaScript code, is create 
   a global object. 
   The properties of this object are the global variables of JavaScript programs. When you declare a global JavaScript variable, what you are actually doing is defining a property of the global object.
The JavaScript interpreter initializes the global object with a number of properties that refer to predefined values and functions. For example, the 
   Infinity, 
   parseInt, and 
   Math properties refer to the number infinity, the predefined 
   parseInt( ) function, and the predefined Math object, respectively.
  In top-level code you can use this to refer to the global object. In a function this has a different use.
  
 
  Like that 
      global variables are properties of the special global object        , 
      local variables are properties of an object         which known as the call object. While the body of a function is executing, the function arguments and local variables are stored as properties of this call object. The use of an entirely separate object for local variables is what allows js to keep local variables from overwriting the value of global variables with the same name.
  
  Each time the js interpreter begins to execute a function; it creates a new 
      execution context         for that function, where js codes execute. An important part of the context is the object in which variables defined. 
  
  Note that js implementations may allow 
      multiple global execution context        , each with a global object, such as client-side javascript, in which each separate browser window, or each frame within a window, defines a separate global execution context.
  
When JavaScript code in one execution context can read and write property values and execute functions that are defined in another execution context, you've reached a level of complexity that requires consideration of 
   security issues.
  

  
   Interpret the topic of variables scope with knowledge of execution context:      
  
  Every JavaScript execution context has a scope chain associated with it. This scope chain is a list or chain of objects. When JavaScript code needs to look up the value of a variable 
      x         (a process called variable name resolution), it starts by looking at the first object in the chain. If that object has a property named 
      x        , the value of that property is used. If the first object does not have a property named 
      x        , JavaScript continues the search with the next object in the chain. If the second object does not have a property named 
      x        , the search moves on to the next object, and so on.
  
    In top-level JavaScript code (i.e., code not contained within any function definitions), the scope chain consists of a single object, the global object. All variables are looked up in this object. If a variable does not exist, the variable value is undefined. In a (nonnested) function, however, the scope chain consists of two objects. The first is the function's call object, and the second is the global object. When the function refers to a variable, the call object (the local scope) is checked first, and the global object (the global scope) is checked second. A nested function would have three or more objects in its scope chain. The figure bellow illustrates the process of looking up a variable name in the scope chain of a function.
    1.4 expressions and operators  
  
  ==equality, ===identity
  
  !==inequlity, !===inidentity
  
   operators:      
  

Table 5-1. JavaScript operators
  P  
  A  
  Operator  
  Operand type(s)  
  Operation performed  
15
L
  .                
object, identifier
Property access

L
  []                
array, integer
Array index

L
  ( )                
function, arguments
Function call

R
  new                
constructor call
Create new object
14
R
  ++                
lvalue
Pre- or post-increment (unary)

R
  --                
lvalue
Pre- or post-decrement (unary)

R
  -                
number
Unary minus (negation)

R
  +                
number
Unary plus (no-op)

R
  ~                
integer
Bitwise complement (unary)

R
  !                
boolean
Logical complement (unary)

R
  delete                
lvalue
Undefine a property (unary)

R
  typeof                
any
Return datatype (unary)

R
  void                
any
Return undefined value (unary)
13
L
  *              , 
       /              , 
       %                
numbers
Multiplication, division, remainder
12
L
  +              , 
       -                
numbers
Addition, subtraction

L
  +                
strings
String concatenation
11
L
  <<                
integers
Left shift

L
  >>                
integers
Right shift with sign extension

L
  >>>                
integers
Right shift with zero extension
10
L
  <              , 
       <=                
numbers or strings
Less than, less than or equal

L
  >              , 
       >=                
numbers or strings
Greater than, greater than or equal

L
  instanceof                
object, constructor
Check object type

L
  in                
string, object
Check whether property exists
9
L
  ==                
any
Test for equality

L
  !=                
any
Test for inequality

L
  ===                
any
Test for identity

L
  !==                
any
Test for nonidentity
8
L
  &                
integers
Bitwise AND
7
L
  ^                
integers
Bitwise XOR
6
L
  |                
integers
Bitwise OR
5
L
  &&                
booleans
Logical AND
4
L
  ||                
booleans
Logical OR
3
R
  ?:                
boolean, any, any
Conditional operator (three operands)
2
R
  =                
lvalue, any
Assignment

R
  *=              , 
       /=              , 
       %=              , 
       +=              , 
       -=              , 
       <<=              , 
       >>=              , 
       >>>=              , 
       &=              , 
       ^=              , 
       |=                
lvalue, any
Assignment with operation
1
L
  ,                
any
Multiple evaluation

  
  delete:
  
Not all variables and properties can be deleted by delete. Some built-in core and client-side properties are immune from deletion, and user defined variables with var can’t be deleted too.
  Note that if a nonexistent property is deleted it returns true.
  
var o = {x:1, y:2};// Define a variable; initialize it to an object
delete o.x; 
    // Delete one of the object properties; returns true
  
typeof o.x; 
    // Property does not exist; returns "undefined"
  
delete o.x; 
    // Delete a nonexistent property; returns true
  
delete o; 
    // Can't delete a declared variable; returns false
  
delete 1; 
    // Can't delete an integer; returns true
  
x = 1; 
    // Implicitly declare a variable without var keyword
  
delete x; 
    // Can delete this kind of variable; returns true
  
x; 
    // Runtime error: x is not defined
  
It is important to understand that delete affects only properties, not objects the properties referred to by those properties. Consider the following code:
var my = new Object( ); 
    // Create an object named "my"
  
my.hire = new Date( ); 
    // my.hire refers to a Date object
  
my.fire = my.hire; 
    // my.fire refers to the same object
  
delete my.hire; 
    // hire property is deleted; returns true
  
document.write(my.fire);// But my.fire still refers to the Date
object
  void:
  
  the purpose of void is to discard its operand value and return undefined.
  
  ()
  
  is a operator, which is used to invoke function.
  
    1.5 statements  
  
 for/in:      
  
  for (variable in object)
  
      statement      
  
 l        execute once for each property of object. And 
   the names of properties is assigned to variable one by one.
  
  for (var prop in my_object) {
  
document.write("name: " + prop + "; value: " + my_object[prop], "<br>");
  }
  
the above code prints the name and value of each property.
 l          Variables in for/in loop can be an expression, as long as it evaluates to something suitable for the left side of an assignment. The expression will be evaluated each time through the loop.
var o = {x:1, y:2, z:3};
var a = new Array( );
var i = 0;
for(a[i++] in o) /* empty loop body */;
the code above is the same as functionally:
a[0]=x;
a[1]=y;
a[2]=z;
 l          when the object in for/in is an array the loop enumerates the array indexes.
  var a= [1,2,3];
  
  for(i in a) alert(i);
  
  the code above is functionally the same as:
  
  i=0;
  
  i=1;
  
  i=2;
  
  0,1,2 are the indexes of the array a.
  
  functions:
  
  function   funcname([  arg1 [,  arg2 [...,   argn]]]) {      
  
    statements  
  
  }      
  
    
  
a function definition creates a new function object and stores that object in a newly created property named funcname.
Technically speaking, the function statement is not a statement. Statements cause dynamic behavior in a javascript program, while function definitions describe the static structure of a grogram. Statements are executed at runtime, functions are defined when js code is parsed, or compiled, before it is actually run. The situation above cause some surprising effects.
alert(f(4)); 
    // Displays 16. f( ) can be called before it 
  
is defined.
var f = 0; 
    // This statement overwrites the property f.
  
function f(x) {// This "statement" defines the function f
before either
  return x*x;// of the lines above are executed.
  
}
alert(f); 
    // Displays 0. f( ) has been overwritten by 
  
the variable f.
  try/catch/finally:
  
   try/finally:      
  
when used above the finally block is simply cleanup code, regardless of break, continue, or return within the try clause

Table 6-1. JavaScript statement syntax
  Statement  
  Syntax  
  Purpose  
break
break;
break 
         label  ;
Exit from the innermost loop or 
       switch               statement or from the statement named by 
         label    
case
case 
         expression  :
Label a statement within a 
       switch               statement
continue
continue;
continue 
         label  ;
Restart the innermost loop or the loop named by 
         label    
default
default:
Label the default statement within a 
       switch               statement
do/while
do
    statement    
while ( 
         expression  );
An alternative to the 
       while               loop
empty
;
Do nothing
for
for ( 
         initialize   ; 
         test   ; 
         increment  )
    statement    
An easy-to-use loop
for/in
for ( 
         variable   in 
         object  )
    statement    
Loop through the properties of an object
function
function 
         funcname  ([ 
         arg1  [..., 
         argn  ]])
{
    statements    
}
Declare a function
if/else
if ( 
         expression  )
    statement1    
[else 
         statement2  ]
Conditionally execute code
label
    identifier  : 
         statement    
Give 
         statement   the name 
         identifier    
return
return [ 
         expression  ];
Return from a function or return the value of 
         expression   from a function
switch
switch ( 
         expression  ) {
    statements    
}
Multiway branch to statements labeled with 
       case               or 
       default:                
throw
throw 
         expression  ;
Throw an exception
try
try {
    statements    
}
catch ( 
         identifier  ) {
    statements    
}
finally {
    statements    
}
Catch an exception
var
var 
         name_1   [ = 
         value_1  ]
[ ,..., 
         name_n   [ = 
         value_n  ]];
Declare and initialize variables
while
while ( 
         expression  )
    statement    
A basic loop construct
with
with ( 
         object  )
    statement    
Extend the scope chain (deprecated)
    1.6 objects and arrays  
  
  Although you can declare a variable with var there is no need to do with object properties. Once you have create an object property by assigning a value to it, you can change the value of properties at any time.
  
 To check the existence of a property         you can use in operator like “if (“propertyname” in objectname)”.
  
  A similar way to check the existence of a property is :
  
If (objectname.propertyname !== undefined). Because if a property exists but has no value it values undefined. The above check that If the property exists and is not undefined.
 
 If you delete a property of an object         using delete, you not merely set the property to undefined but also remove it from the object.
  
   The difference between object.property and object[“property”]:      
  
    The former use an indentity(property) and the latter uses a string. 
   Identities can’t be manipulated but strings can.       That is that in c or so on the number of property in an object is fixed, but it is not true in javascript. What’s more, if the property is an identity it must be typed literally into your program. You can manipulate the properties which are strings, which has great flexibility.
var addr = "";
for(i = 0; i < 4; i++) {
  addr += customer["address" + i] + '/n';
  
}
  All objects in javascript inherit from the Object class.      
  
  The property and methods in Ojbect:
  
  l          constructorproperty:      
  
theconstructor property refers to the constructor function that initialize the object.
  var d = new Date();
  
d.constructor == Date;// Evaluates to true
   also,       instanceof operator exists.
 
  l          toString():      
  
  l          toLocalString():      
  
the default implementation doesn’t do any localization. It returns the same as toString().
 l          hasOwnProperty():      
  
to check that if the object define a noninherited property of the specific name .
  l          propertyIsEnumarable():      
  
  l          isPrototypeOf():      
  

  

  
  you can define nonnumeric object properties on an array and access them using the . or [] syntax:      
  
    a[-1.23] = true;
  because -1.23 can be an index obviously it is converted to a string “-1.23” and named a property which has a boolean value true.
  
arrays may be sparse which means that array indexes need not to be a continues range of numbers.
  delete operator only sets an array element to undefined not remove it from the array. To actually delete an element, so that all elements above it are shifted down to lower indexes, you must use an array method. Array.shift() deletes the first element of an array, Array.pop() deletes the last element, and Array.splice() deletes a contiguous range of elements from an array.
  Length of array:
  
var a = new Array(); 
    // a.length == 0(no elements defined)
  
a = new Array(10); 
    // a.length == 10 (empty elements 0-9 defined)
  
a = new Array(1,2,3);// a.length == 3(elements 0-2 defined)
a = [4, 5]; 
    // a.length == 2(elements 0 and 1 defined)
  
a[5] = -1; 
    // a.length == 6(elements 0, 1, and 5 defined)
  
a[49] = 0; 
    // a.length == 50 (elements 0, 1, 5, and 49 defined)
  
  length property of an array is 
   read/write value.
  so setting length to a smaller value than its current value truncates the array to the new value and some elements are discarded. If you set a larger value new undefined values are added to increase the array to the new size.
  
  Js doesn’t support muiltidimensional arrays, but it does allow you to 
   approximate them quite nicely with arrays of arrays.
  Array methods:
  
 l        join():
convert all the elements of an array to strings and concatenate them:
var a = [1, 2, 3]; 
    // Create a new array with these three 
  
elements
var s = a.join(); 
    // s == "1,2,3"
  
Note:
var t = a.join(“, ”); // t==”1, 2, 3”
use “, ” as the delimiter.
 l        revers():
 l        sort():
  no arguments: sort alphabetically. 
  
  Use a function as the argument: sort by some order selfdefined:
  
var a = [33, 4, 1111, 222];
a.sort(); 
    // Alphabetical order:1111, 222, 33, 4
  
a.sort(function(a,b) { 
    // Numerical order: 4, 33, 222, 1111
  
  return a-b; // Returns < 0, 0, or > 0, depending on order
  
  });
  
 l        concate():
if the arguments are arrays they are flattend and then added to the array.
var a = [1,2,3];
a.concat(4, 5) 
    // Returns [1,2,3,4,5]
  
a.concat([4,5]); 
    // Returns [1,2,3,4,5]
  
a.concat([4,5],[6,7]) 
    // Returns [1,2,3,4,5,6,7]
  
a.concat(4, [5,[6,7]])// Returns [1,2,3,4,5,[6,7]]
[5,[6,7]] in the last statement is flatted as 
   5,[6,7], so [5,[6,7]] is added and the returned array is [1,2,3,4, 
   5,[6,7]]
 l        slice():
if the argument is negative it means the offset from the last element.
var a = [1,2,3,4,5];
a.slice(0,3); 
    // Returns [1,2,3]
  
a.slice(3); 
    // Returns [4,5]
  
a.slice(1,-1); 
    // Returns [2,3,4]
  
a.slice(-3,-2);// Returns [3]
 l        splice():
  it can insert and remove elements from an array.
  
 l        push(), pop():
  work with arrays as if they are stacks.
  
 l        unshift(), shift():
  work with arrays as if they are queues.
  
 l        toString(), toLocalString():
note that the result doesn’t contain any square brackets or any other delimiter:
[1,2,3].toString() 
    // Yields '1,2,3'
  
["a", "b", "c"].toString()// Yields 'a,b,c'
[1, [2,'c']].toString() 
    // Yields '1,2,c'
  
 l        methods for firefox:
  indexOf, lastIndexOf(), forEach(), map(), filter()
  
It is often perfectly reasonable to treat any object with a length property and corresponding nonnegative integer properties as a kind of array. These “array-like” objects actually do occasionally appear in practice although you can’t invoke array methods on them or except special behavior from the length property, you can still iterate through them with the same code you’d use for a true array. As long as you don’t add elements or change the length property you can treat an array-like object as a real array.
var a = {};// Start with a regular empty object
// Add properties to make it "array-like"
var i = 0;
while(i < 10) {
  a[i] = i * i;
  
  i++;
  
}
a.length = i;
// Now iterate through it as if it were a real array
var total = 0;
for(var j = 0; j < a.length; j++)
  total += a[j];
  
note that In client-side JavaScript, a number of DOM methods, such as 
   document.getElementsByTagName(), return array-like objects.
  1.7 functions      
  
  1.7.1 basis      
  
  function literals:
  
  unnamed: 
  
var f = function(x) { return x*x; };
named(in order to recurs etc. ):
var f = function fact(x) { if (x <= 1) return 1; else return
x*fact(x-1); };
  function statements:
  
  function f(x) { return x*x; }
  
 
  because function literals are created by js expressions they are quite flexible and particularly well suited for functions that are used only once and need not to named.
 
  When a function invoked with fewer arguments than declared the additional arguments have the undefined value.
  
  The arguments object within a function:
  
  There is an Arguments object named arguments within the body of a function. The Arguments object is an array-like object. Although functions declare a fixed number of arguments it can be passed any number of arguments. The Arguments object allows full access to these argument values, even when some or all are unnamed. If you define a function with one arguments but you call it with two. You can access the second argument by arguments[1]. Furthermore, arguments has a length property, like true arrays.
  
 The arguments can be used in many ways.      
  
  To verify that a function is invoked with the correct number of arguments:
  
function f(x, y, z)
{
  // First, verify that the right number of arguments was passed
  
  if (arguments.length != 3) {
  
  throw new Error("function f called with " + arguments.length +
  
  "arguments, but it expects 3 arguments.");
  
  }
  
  // Now do the actual function...
  
}
  variadic functions:
  
with arguments it is possible that: 
   js functions can be written so that they   work with any number of arguments      . This kind of functions are called variadic functions.
There is an example that shows how to find out the biggest number from any number of numbers:
 function max(/* ... */)
  
 {
  
  var m = Number.NEGATIVE_INFINITY;      
  
  // Loop through all the arguments, 
      
  
 looking for, and
  
  // remembering, the biggest      
  
  for(var i = 0; i < arguments.length; i++)      
  
  if (arguments[i] > m) m = arguments[i];      
  
  // Return the biggest      
  
  return m;      
  
 }
  
 var largest = max(1, 10, 100, 2, 3, 1000, 4, 5, 10000, 6);
  
 now you can see that you can’t imagine how many numbers will be compared. With arguments you don’t need to care about the number of arguments that functions need.
  
  arguments is not an array. It is an object.      
  
    It defides numbered array elements and a length property.
  arguments is not a reserved word       so it can be hided if you define your own arguments variable. You’d better treat it as a reserved word.
  calleeproperty:
  
  arguments has a callee property referring to the function. It can be used to invoke an unnamed function itself recursively.
  
Because javascript is an untyped and flexible language sometimes it is better to do some type verifying when the value you need is some specific type data.
  1.7.2 functions as data      
  
Functions in js can also be data besides syntax. It means that functions can be assigned to variables, stored in the properties of objects or the elements of arrays, passed as arguments to functions, and so on.
 l        We can look functions as below:
  functionsquare(x){return x*x;}
  
  the code creates a new function object and assigns it to the variable square. The function can also be assigned to another variable:
  
var b = square;// Now b refers to the same function that square
does
var c = b(5); // c contains the number 25
 l          functions can also be assigned to object properties.
  
var o = new Object;
o.square = function(x) { return x*x; } 
    // function literal
  
y = o.square(16); 
    // y equals 256
  
 l          functions don’t even require names at all when they are assigned to array elements:
  
var a = new Array(3);
a[0] = function(x) { return x*x; }
a[1] = 20;
a[2] = a[0](a[1]); 
    // a[2] contains 400
  
 l          passing a function as an argument to other functions.
  
  The codes following are quite different in behaviors compared with java, c++ and so on.      
  
// We define some simple functions here
function add(x,y) { return x + y; }
function subtract(x,y) { return x - y; }
function multiply(x,y) { return x * y; }
function divide(x,y) { return x / y; }
// Here's a function that takes one of the above functions
// as an argument and invokes it on two operands
function operate(operator, operand1, operand2)
{
  return operator(operand1, operand2);
  
}
// We could invoke this function like this to compute the value (2+3) + (4*5):
var i = operate(add, operate(add, 2, 3), operate(multiply, 4, 5));
// For the sake of the example, we implement the simple functions again, this time
// using function literals within an object literal;
var operators = {
  add: function(x,y) { return x+y; },
  
  subtract: function(x,y) { return x-y; },
  
  multiply: function(x,y) { return x*y; },
  
  divide: function(x,y) { return x/y; },
  
  pow: Math.pow// Works for predefined functions too
  
};
// This function takes the name of an operator, looks up that operator
// in the object, and then invokes it on the supplied operands. Note
// the syntax used to invoke the operator function.
function operate2(op_name, operand1, operand2)
{
  if (typeof operators[op_name] == "function")
  
  return operators[op_name](operand1, operand2);
  
  else throw "unknown operator";
  
}
// We could invoke this function as follows to compute
// the value ("hello" + " " + "world"):
var j = operate2("add", "hello", operate2("add", " ", "world"))
// Using the predefined Math.pow() function:
var k = operate2("pow", 10, 2)

  
  1.7.3 functions as methods      
  
When a function is assigned to a property of an object, it is often referred to as a 
   method         of that object.
  
 l        To assign a function to a property:
Object.property = function;
To invoke:
Object.property();
 l          To use “this” in method to refer to the object that contains the method:
var calculator = {// An object literal
  operand1: 1,
  
  operand2: 1,
  
  compute: function() {
  
  this.result = this.operand1 + this.operand2;
  
  }
  
};
calculator.compute(); 
    // What is 1+1?
  
print(calculator.result); 
    // Display the result
  
 l          when use “this” in a function which is not invoked as a method “this” refer to the global object. Even when “this” exists in the nested functions which is contained by a method.
  1.7.4 as an object functions properties and methods      
  
 l        the 
   length property of the function itself is read only. It returns the number of arguments excepted.
 l        The 
   prototype property:
 l        Defining 
   your own properties:
When a function need a variable whose value persists across invocation you can use a property of the Function object instead of defining a global variable.
// Create and initialize the "static" variable.
// Function declarations are processed before code is executed, so
// we really can do this assignment before the function declaration.
uniqueInteger.counter = 0;
// Here's the function. It returns a different value each time
// it is called and uses a "static" property of itself to keep track
// of the last value it returned.
function uniqueInteger() {
  // Increment and return our "static" variable
  
  return uniqueInteger.counter++;
  
}
 l          apply() and call()
  
  1.7.5 functions scope and closures      
  
  Functions are lexically. This means that they run in the scope in which they are defined, not the scope from which they are executed. When a function is defined, the current scope chain is saved and becomes part of the internal state of the function. At the top level, the scope chain simply consists of the global object, and lexical scoping is not particularly relevant. When you define a nested function, however, the scope chain includes the containing function. This means that nested functions can access all of the arguments and local variables of the containing function.
  
  When the javascript interpreter invokes a function, it first sets the scope to the scope chain that was in the effect when the function was defined. Next it adds a new object known as the 
      call         object to the front of the scope chain. The call object is initialized with a property named 
      arguments        . so arguments of the function are added into the arguments object and local variables declared with the var statement are also defined within the call object.
  
 It is sometimes useful to define a function simply to create a call object.         The call object acts as a temporary namespace where you can define variables and create properties without corrupting the global namespace.
  
  If there a piece of code you want to use in other pieces of codes. But there are variables acting as intermediate results. The problem is that since this code will be used in many different programs, you don't know whether the variables it creates will conflict with variables used by the programs that import it. So the solution is to put that piece of code into a function and then invoke it.
  
 Considering the code following:      
  
// This function returns a function each time it is called
// The scope in which the function is defined differs for each call
function makefunc(x) {
  return function() { return x; }
  
}
// Call makefunc() several times, and save the results in an array:
var a = [makefunc(0), makefunc(1), makefunc(2)];
// Now call these functions and display their values.
// Although the body of each function is the same, the scope is
// different, and each call returns a different value:
alert(a[0]());// Displays 0
alert(a[1]());// Displays 1
alert(a[2]());// Displays 2
  when a function is invoked a call object is created and placed on the scope chain. When the function exists the call object is removed from the scope chain. When no nested functions are involved, the scope chain is the only reference to the call object. When the object is removed from the chain, there are no more references to it and it is garbage collected.但是當(dāng)有內(nèi)嵌的函數(shù)時(shí)情況有點(diǎn)不同。當(dāng)一個(gè)內(nèi)嵌函數(shù)被生成時(shí)，該函數(shù)的定義就會(huì)指向call對象，因?yàn)閏all對象在scope chain的頂端。如果內(nèi)嵌的函數(shù)只在外部的函數(shù)中使用的話，那么指向內(nèi)嵌函數(shù)的引用只能在call對象中找到。當(dāng)外部函數(shù)返回，內(nèi)嵌函數(shù)指向call，call指向內(nèi)嵌函數(shù)，沒有其他的引用了。因此，這兩者都會(huì)被垃圾收集。
  1.7.6 the Function() constructor      
  
  a function can also be defined with the Function() constructor like:
  
  var f = new Function(“x”, “y”, “return x*y”);
  
  the code above is the same as functionally:
  
  function f(x,y){return x*y;}
  
 
  The Function() constructor has any number of string arguments and the last one is the body of the function. Note that the function created by the Function() has no name.
  
  Function() constructor allows the function code to be dynamically created and compiled at runtime.
  
  Function() constructor parses the function code and create a new function object each time it is called. So if the Function() exists within a loop or a frequently called code and Function() may be called for many times. It is inefficient. By contrast, a function literal or nested function is not recompiled every time.
  
  The functions the Function() creates don’t use lexical scoping. They are compiled as if they were in top-level.
  
var y = "global";
function constructFunction() {
  var y = "local";
  
  return new Function("return y");// Does not capture the local scope!
  
}
// This line displays "global" because the function returned by the
// Function() constructor does not use the local scope. Had a function
// literal been used instead, this line would have displayed "local".
alert(constructFunction 
   ()());// Displays "global"
  pay attention to the two “()”. If you write constructFunction() it displays :
  
  function annoymouse(){
  
  return y;
  
}
  1.8 classes, constructors and prototypes      
  
  1.8.1 constructors      
  
example codes：
// Define the constructor.
// Note how it initializes the object referred to by "this".
function Rectangle(w, h) {
  this.width = w;
  
  this.height = h;
  
  // Note: no return statement here
  
}
// Invoke the constructor to create two Rectangle objects.
// We pass the width and height to the constructor
// so that it can initialize each new object appropriately.
var rect1 = new Rectangle(2, 4); 
    // rect1 = { width:2, height:4 };
  
var rect2 = new Rectangle(8.5, 11); // rect2 = { width:8.5, height:11 };
Note that the constructor usually doesn’t return a value. If you return an object value the returned value becomes the value of the new expression. In this case the object that was the value of this is discard.
  1.8.2 prototypes and inheritance      
  
  We can assign a function to a property of an object. So the function is a method, just like:
  
function Rectangle(w, h) {
  this.width = w;
  
  this.height = h;
  
  this.area = function( ) { return this.width * this.height; }
  
}
of course, width and height properties are always different. But the method area is always referred to the same function. It is inefficient to use regular properties for methods that are intended to be shared by all objects of the same class. And sometimes the method may want to be changed.
There is an internal reference to each object’s prototype object within every object.
When a function is defined there is a prototype property automatically created and initialized. The prototype property is that is an object with a property named constructor initially which refers back to the constructor function with which the prototype is associated. Any properties added to this prototype object will appear t be properties of objects initialized by the constructor.
// The constructor function initializes those properties that
// will be different for each instance.
function Rectangle(w, h) {
  this.width = w;
  
  this.height = h;
  
}
  // The prototype object holds methods and other properties that      
  
  // should be shared by each instance.      
  
Rectangle.prototype.area = function( ) { return this.width * this.height; }
The prototype object is associated with the constructor, and each object initialized by the constructor inherits the same set of properties from the prototype. It means that the prototype object is an ideal place for methods and other constant properties.
Note that although inheriting happens properties are not copied from the prototype object into new objects. They appears as if they were properties of new objects.
  It has two important implications.
  
  The use of prototype objects can dramatically decrease the amount of memory required by each object because the object can inherit many of its properties.
  
  You can add new methods to existing classes which is not a good idea.
  
The discussion following is from core javascript guide 1.5
  http://devedge-temp.mozilla.org/library/manuals/2000/javascript/1.5/guide/index.html
  

  Objects and arrays are different interfaces to the same data structure.                
  To create an object using a constructor function:                
1 define the object type by writing a constructor function.
2 create an instance of the object with new.
Note that you can add a new property to an instance at anytime like:
car2.newProperty = newValue;
But note that the new property merely affect the instance car2 not any other instances of the same object type. If you want to add a new property to an object type you should manipulate on the object type.
You can refer a property by name or ordinal index. But if you define a property with a name you may always refer to it by name, and if you define a property with an index you must always refer to it by an index.
  Class-based versus prototype-based languages  
For class-based languages 
       there is a distinction between classes and instances              .
For prototype-based languages there is not.
  They simply have objects              .
And there is concept of a 
       prototype               object which used as a template to get initial properties for a new object.
  Any object can be the prototype of another object. 
                
Any object can 
       specify its own properties at any time 
              whether you create it or run it.
In classed-based languages there is a separate fragment of class defining where you can specify constructor methods. A constructor can specify initial values for the instance’s properties and perform other processing appropriate at creation time.
In javascript you define a constructor function to create objects with a particular initial set of properties and values. 
       Any javascript function can be used as a constructor. You use new operator with a constructor with a constructor function to create to new object.
  Inheritance                
Javascript implements 
       inheritance by allowing you to associate a prototypical object with any constructor function              . For example, you define the Employee constructor function with some properties. Next, you define the Manager constructor function with its own properties. Finally, you assign a new Employee object as the prototype for the Manager constructor function. Then when you create a new Manager, it inherits the properties from the Employee object.
  Adding and removing properties  
In class-based languages you can’t change the number of properties after your definition. But you can do it in javascript.
  Table 8.1 Comparison of class-based (Java) and prototype-based (JavaScript) object systems 
                
 Class-based (Java) 
                          Prototype-based (JavaScript) 
                          Class and instance are distinct entities.     All objects are instances.     Define a class with a class definition; instantiate a class with constructor methods.     Define and create a set of objects with constructor functions.     Create a single object with the new operator.     Same.     Construct an object hierarchy by using class definitions to define subclasses of existing classes.     Construct an object hierarchy by assigning an object as the prototype associated with a constructor function.     Inherit properties by following the class chain.     Inherit properties by following the prototype chain.     Class definition specifies 
                      all                         properties of all instances of a class. Cannot add properties dynamically at run time.     Constructor function or prototype specifies an 
                      initial set                         of properties. Can add or remove properties dynamically to individual objects or to the entire set of objects.   
There is an example which shows the differences between java and javascript.
    
to implements the hierarchy above we do in javascript:
    
what’s more:
 JavaScript 
                          Java 
                          function Employee () {                        this.name = "";                        this.dept = "general";                        }     public class Employee {                        public String name;                        public String dept;                        public Employee () {                        this.name = "";                        this.dept = "general";                        }                        }     JavaScript 
                          Java 
                          function Manager () {                        this.reports = [];                        }                        Manager.prototype = new Employee;     function WorkerBee () {                        this.projects = [];                        }                        WorkerBee.prototype = new Employee;     public class Manager extends Employee {                        public Employee[] reports;                        public Manager () {                        this.reports = new Employee[0];                        }                        }     public class WorkerBee extends Employee {                        public String[] projects;                        public WorkerBee () {                        this.projects = new String[0];                        }                        }     JavaScript 
                          Java 
                          function SalesPerson () {                        this.dept = "sales";                        this.quota = 100;                        }                        SalesPerson.prototype = new WorkerBee;     function Engineer () {                        this.dept = "engineering";                        this.machine = "";                        }                        Engineer.prototype = new WorkerBee;     public class SalesPerson extends WorkerBee {                        public double quota;                        public SalesPerson () {                        this.dept = "sales";                        this.quota = 100.0;                        }                        }     public class Engineer extends WorkerBee {                        public String machine;                        public Engineer () {                        this.dept = "engineering";                        this.machine = "";                        }                        }   
Note that 
       “instance” in javascript does not have the same meaning as in class-based languages. In javascript we talk about “instance” as an object created using particular constructor function. So in the examples above jane is an instance of Engineer.
Note that although the terms parent, child, ancestor, and descendant do not have formal meanings in JavaScript; 
        you can use them informally to refer to objects higher or lower in the prototype chain.
  The following section shows that how an new object is created in more detail              :
Mark = new WorkerBee;
 1.                In javascript, the new operator 
       creates a new generic object and javascript 
       passess this new object as the value of the this keyword to the WorkerBee constructor function.
 2.                Then the constructor function 
       sets the value of the new object’s own properties. It also sets an internal 
       _proto_ property to the value of WorkerBee.prototype. the _proto_ property determines the prototype chain used to return property values.
 3.                Then javascript returns the new object and sets the mark to the returned object.
 4.                javascript doesn’t explicitly put values that is herited from the prototype chain. When you ask for the value of a property, javascript will first check to see if the value exists in that object. if not to check from upper level in the prototype chain using the _proto_ property. And so on.
More about adding property
If you add a new property to an object that is used as the prototype for a constructor function, you add that property to all objects that inherit from it. For example,
    
  how to specify initial values to the inherited properties:                
for example:
function Engineer (name, projs, mach) { 
     
this.base = WorkerBee; 
     
this.base(name, "engineering", projs); 
     
this.machine = mach || ""; 
     
}
if you create an object like:
jane = new Engineer("Doe, Jane", ["navigator", "javascript"], "belau");
then something happen as these steps:
  1.                  ?The new operator creates a generic object and sets its __proto__ property to Engineer.prototype.   
  2.                  ?The new operator passes the new object to the Engineer constructor as the value of the this keyword.   
  3.                  ?The constructor creates a new property called base for that object and assigns the value of the WorkerBee constructor to the base property. This makes the WorkerBee constructor a method of the Engineer object. The name of the base property is not special. You can use any legal property name; base is simply evocative of its purpose.   
  4.                  ?The constructor calls the base method, passing as its arguments two of the arguments passed to the constructor ("Doe, Jane" and ["navigator", "javascript"]) and also the string "engineering". Explicitly using "engineering" in the constructor indicates that all Engineer objects have the same value for the inherited dept property, and this value overrides the value inherited from Employee.   
  5.                  ?Because base is a method of Engineer, within the call to base, JavaScript binds the this keyword to the object created in 
              Step 1                . Thus, the WorkerBee function in turn passes the "Doe, Jane" and ["navigator", "javascript"] arguments to the Employee constructor function. Upon return from the Employee constructor function, the WorkerBee function uses the remaining argument to set the projects property.   
  6.                  ?Upon return from the base method, the Engineer constructor initializes the object's machine property to "  belau  ".   
 7.                  ?Upon return from the constructor, JavaScript assigns the new object to the jane variable.  
  If you want to change the value of an object property at run time and have the new value be inherited by its descendants, you can’t define the property in the object’s constructor function. Because if you do so, you can see:  
  function obj1()  
  {  
  this.pro=””;  
  }  
function obj2()
{}
obj2.prototype=new obj1;
Now you can see that there is an instance of obj1 assigned to obj2.prototype. It means that there is a local variabl prototype. So the pro property of obj2.prototype exists. If you change the pro of obj1, no affection may be made on obj2. And you can resolve this problem as:
function obj1()
{}
obj1.pro=””;
function obj2()
{}
obj2.prototype=new obj1;
In the case above if you change the pro property of obj1 the pro property inherited by obj2 from obj1 is also changed.
  There is an example of prototype chain.                
chris = new Engineer("Pigman, Chris", ["jsd"], "fiji");
With this object, the following statements are all true:
chris.__proto__ == Engineer.prototype; 
     
chris.__proto__.__proto__ == WorkerBee.prototype; 
     
chris.__proto__.__proto__.__proto__ == Employee.prototype; 
     
chris.__proto__.__proto__.__proto__.__proto__ == Object.prototype; 
     
chris.__proto__.__proto__.__proto__.__proto__.__proto__ == null;
  javascript 學(xué)習(xí)
  

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